Class 12 Biology · NCERT chapter 8 · 7 topics · free to read
Common Human Diseases
Diseases caused by pathogens — bacteria (typhoid, pneumonia, plague), viruses (common cold, dengue), protozoans (malaria, amoebiasis), and helminths (ascariasis, filariasis). Each pathogen has a specific mode of transmission, organ affected, and diagnostic method.
Bacteria cause some of the most common NEET-tested diseases. Each has a specific causative organism, mode of transmission, and target organ.
Bacterial diseases are caused by pathogenic bacteria that invade tissues, multiply, and produce toxins. The four major bacterial diseases in NCERT are typhoid (Salmonella typhi), pneumonia (Streptococcus pneumoniae / Haemophilus influenzae), plague (Yersinia pestis), and diphtheria (Corynebacterium diphtheriae).
Typhoid — Caused by *Salmonella typhi*. Enters through contaminated food/water. Pathogen reaches small intestine → enters blood → spreads to organs. Widal test confirms diagnosis. Mary Mallon ('Typhoid Mary') was a famous carrier
Pneumonia — Caused by *Streptococcus pneumoniae* (bacterial) or *Haemophilus influenzae* (Type b). Infects alveoli of lungs → alveoli fill with fluid → difficulty breathing. Spreads by droplet infection (coughs/sneezes) or sharing utensils
Plague — Caused by *Yersinia pestis*. Transmitted by rat flea (vector). Three forms: bubonic (lymph nodes swell), septicemic (blood), pneumonic (lungs)
Diphtheria — Caused by *Corynebacterium diphtheriae*. Affects throat → forms a tough membrane → can block airway. Part of DPT vaccine
Viral Diseases
Viruses are obligate intracellular parasites. They hijack host cell machinery to replicate. Major viral diseases include common cold, dengue, and chikungunya.
Viruses cannot reproduce on their own — they need a living host cell. Once inside, they redirect the cell's machinery to make copies of themselves, often destroying the cell in the process.
Common Cold — Caused by Rhinoviruses. Infects nose and respiratory passage (NOT lungs). Symptoms: nasal congestion, sore throat, cough. Spreads by droplets or contaminated objects. No vaccine exists (too many strains)
Dengue — Caused by Dengue virus. Vector: Aedes aegypti mosquito. Symptoms: high fever, severe joint pain ('breakbone fever'), rash. Severe form: dengue hemorrhagic fever
Chikungunya — Caused by Chikungunya virus. Vector: Aedes mosquito. Symptoms similar to dengue but with more joint pain
Caused by Rhinovirus
Caused by Streptococcus / Haemophilus
Affects nose & upper respiratory
Affects lungs (alveoli)
Mild — self-limiting
Serious — can be fatal
No vaccine available
Vaccine available (PCV)
Virus
Bacteria (usually)
Protozoan Diseases
Protozoans are unicellular eukaryotes that cause some of the world's deadliest diseases. Malaria alone kills ~600,000 people per year.
Protozoan parasites are single-celled eukaryotes that have complex life cycles, often involving multiple hosts. They evade the immune system by changing surface antigens or hiding inside cells.
Malaria — Caused by *Plasmodium* (P. vivax, P. falciparum, P. malariae, P. ovale). Vector: female Anopheles mosquito. Detailed life cycle covered in next concept
Amoebiasis (Amoebic dysentery) — Caused by *Entamoeba histolytica*. Transmitted through contaminated food/water. Affects large intestine → causes abdominal pain, bloody stools with mucus. Houseflies act as mechanical carriers
African Sleeping Sickness — Caused by *Trypanosoma brucei*. Vector: Tsetse fly (*Glossina*). Affects nervous system → causes lethargy, sleep disturbance, coma
Kala-azar (Visceral Leishmaniasis) — Caused by *Leishmania donovani*. Vector: Sandfly. Affects liver, spleen, bone marrow
Helminth Diseases
Helminths (parasitic worms) cause chronic debilitating diseases. They have complex life cycles and can live in the host for years.
Helminth parasites are multicellular worms that live inside the host body, feeding on nutrients and blood. They're broadly classified into roundworms (nematodes) and flatworms (cestodes and trematodes).
Ascariasis — Caused by *Ascaris lumbricoides* (roundworm). Transmitted through contaminated food/water/soil. Lives in small intestine → causes abdominal pain, nausea, intestinal blockage. Most common helminth infection worldwide
Filariasis (Elephantiasis) — Caused by *Wuchereria bancrofti* and *W. malayi*. Vector: female Culex mosquito. Worms live in lymphatic vessels → block lymph drainage → chronic swelling of legs, arms, scrotum
Taeniasis — Caused by *Taenia solium* (pork tapeworm). Transmitted through undercooked pork. Lives in small intestine → absorbs digested food. Can grow up to 10 meters
Ringworm — Despite the name, caused by fungi (*Microsporum*, *Trichophyton*, *Epidermophyton*), NOT a worm. Affects skin, nails, scalp. Appears as dry, scaly ring-shaped lesions
Master Disease Table
A single comprehensive table covering every disease, its pathogen, transmission mode, affected organ, and diagnostic test — the ultimate NEET revision tool.
NEET questions on diseases are often pure recall — match the disease to its pathogen, vector, or affected organ. This table covers everything you need.
Typhoid — *Salmonella typhi* | Contaminated food/water | Small intestine → blood → organs | Widal test
Ringworm — *Microsporum / Trichophyton* | Direct contact, fomites | Skin, nails, scalp | KOH mount
This table alone can answer 3-4 NEET questions. Screenshot it, stick it on your wall, review it every day during the last month of prep.
Malaria (Detailed)
Malaria is caused by Plasmodium species transmitted by the female Anopheles mosquito. The parasite has a complex life cycle involving both human (asexual) and mosquito (sexual) stages, with characteristic cyclic fever caused by synchronous RBC rupture.
Four species of Plasmodium infect humans: P. vivax, P. falciparum, P. malariae, and P. ovale. Each causes a distinct fever pattern and severity.
Malaria is one of the oldest and deadliest diseases in human history. It kills about 600,000 people per year, mostly children under 5 in sub-Saharan Africa. The word 'malaria' literally means 'bad air' (Italian: *mala aria*) — people once thought it came from swamp fumes.
P. vivax — Most common in India. Causes benign tertian malaria (fever every 48 hours). Can form dormant liver stages (hypnozoites) → causes relapses months/years later
P. falciparum — Most dangerous/lethal. Causes malignant tertian malaria (fever every 48 hours but irregular). Can cause cerebral malaria — parasitized RBCs block brain capillaries. No hypnozoites
P. malariae — Causes quartan malaria (fever every 72 hours). Mildest form. No hypnozoites
P. ovale — Rare. Causes benign tertian malaria (like P. vivax). Forms hypnozoites
Life Cycle in Humans
In humans, Plasmodium undergoes asexual reproduction in two phases: the liver stage (exo-erythrocytic) and the blood stage (erythrocytic). The blood stage causes all the symptoms.
When an infected female Anopheles mosquito bites you, she injects sporozoites (the infective form) from her salivary glands into your blood. The life cycle then proceeds in two stages inside the human body.
Step 1: Sporozoites enter blood — Injected by female Anopheles mosquito during a blood meal
Step 2: Liver stage (Exo-erythrocytic cycle) — Sporozoites travel to liver within 30 minutes → enter liver cells (hepatocytes) → multiply by schizogony → produce thousands of merozoites. This stage is silent — no symptoms
Step 3: Blood stage (Erythrocytic cycle) — Merozoites released from liver enter RBCs → multiply inside RBCs → RBCs burst (lyse) → release more merozoites + toxins (hemozoin). Each burst = fever spike
Step 4: Cycle repeats — Released merozoites infect new RBCs → multiply → burst → infect more RBCs. This creates the cyclic fever pattern
Step 5: Gametocyte formation — Some merozoites develop into male and female gametocytes inside RBCs. These don't cause symptoms — they wait to be picked up by the next mosquito
Life Cycle in Mosquito
The mosquito is the definitive host where Plasmodium undergoes sexual reproduction. Gametocytes form gametes → fertilization → oocyst → sporozoites that migrate to salivary glands.
When a female Anopheles mosquito bites an infected person, she ingests blood containing gametocytes. Inside the mosquito gut, the sexual cycle begins — this is why the mosquito is the definitive host (sexual reproduction) and humans are the intermediate host (asexual reproduction).
Step 1: Female Anopheles ingests gametocytes (male + female) with the blood meal
Step 2: Gametogenesis — In the mosquito gut, gametocytes mature into gametes. Male gametocyte → 8 microgametes (by exflagellation). Female gametocyte → 1 macrogamete
Step 3: Fertilization — Microgamete fuses with macrogamete → forms zygote (the ONLY diploid stage in the entire life cycle)
Step 4: Ookinete — Zygote becomes motile → called ookinete → penetrates mosquito gut wall
Step 5: Oocyst — Ookinete forms oocyst on outer surface of gut → undergoes sporogony (meiosis + mitosis) → produces thousands of sporozoites
Step 6: Oocyst bursts → sporozoites migrate to salivary glands → ready to infect the next human
Quick recall
Who is the definitive host for Plasmodium — human or mosquito?
The female Anopheles mosquito is the definitive host because sexual reproduction (fertilization) occurs there. Humans are the intermediate host (asexual reproduction only). NEET tests this concept frequently.
Diagnosis, Treatment & Prevention
Malaria is diagnosed by blood smear microscopy or rapid diagnostic tests. Prevention involves vector control (bed nets, insecticides) and prophylactic drugs.
Early diagnosis is critical — especially for P. falciparum malaria, which can progress to cerebral malaria and death within 24-48 hours. The gold standard for diagnosis is microscopic examination of Giemsa-stained blood smears.
Diagnosis: Blood smear (thick + thin film), Rapid Diagnostic Tests (RDTs), PCR for species identification
Prevention — Personal: Insecticide-treated bed nets (ITNs), mosquito repellents, wearing full-sleeve clothing at dusk/dawn
Prevention — Community: Indoor residual spraying (IRS), draining stagnant water (breeding sites), larvicides, biological control (Gambusia fish eat mosquito larvae)
Vaccine: RTS,S/AS01 (Mosquirix) — world's first malaria vaccine. ~30% efficacy. WHO recommended for children in high-risk areas (2021)
Immunity
Immunity is the body's ability to fight pathogens. It has two arms: innate (non-specific, present from birth) and adaptive (specific, acquired after exposure). Adaptive immunity is further divided into humoral (B-cells, antibodies) and cell-mediated (T-cells). Vaccination exploits immunological memory to provide long-lasting protection.
Innate immunity is non-specific, present from birth, and acts immediately against any pathogen. It includes physical barriers (skin, mucus), chemical barriers (lysozyme, HCl), cellular barriers (phagocytes, NK cells), and the inflammatory response.
Innate immunity is your body's first and fastest response to any invader. It doesn't distinguish between pathogens — it attacks everything that's 'non-self.' Think of it as the security guards at a building entrance: they don't check your identity, they just stop anyone who looks suspicious.
Chemical barriers — Lysozyme in tears, saliva, sweat (breaks bacterial cell walls), HCl in stomach (pH ~1.5-2, kills most ingested microbes), sebum on skin (antimicrobial fatty acids)
Cellular barriers — Neutrophils (most abundant WBC, first responders), Monocytes/Macrophages (engulf pathogens by phagocytosis), Natural Killer (NK) cells (destroy virus-infected cells and tumor cells without prior sensitization)
Cytokine barriers — Interferons (proteins released by virus-infected cells that protect neighboring cells), Complement system (cascade of proteins that punch holes in bacterial membranes)
Adaptive (Acquired) Immunity
Adaptive immunity develops after exposure to a specific antigen. It is pathogen-specific, has memory, and gets stronger with each subsequent exposure. Two types: humoral (B-cells → antibodies) and cell-mediated (T-cells).
Unlike innate immunity, adaptive immunity is tailored to each specific pathogen. It takes time to develop (days to weeks for first exposure) but creates memory cells that enable a faster, stronger response if the same pathogen attacks again. This is the principle behind vaccination.
Humoral immunity (Antibody-Mediated Immunity/AMI) — Mediated by B-lymphocytes. B-cells produce antibodies that circulate in blood and body fluids ('humor' = fluid). Antibodies neutralize toxins, coat pathogens for phagocytosis (opsonization), and activate complement
Cell-mediated immunity (CMI) — Mediated by T-lymphocytes. T-cells directly attack infected cells. Cytotoxic T-cells (Tc/CD8+) kill virus-infected cells and cancer cells. Helper T-cells (Th/CD4+) coordinate the entire immune response by releasing cytokines
Primary response — First exposure to antigen → slow (7-10 days), low antibody titer, mainly IgM produced, generates memory cells
Secondary (Anamnestic) response — Second exposure → fast (1-2 days), high antibody titer (100-1000x), mainly IgG produced, lasts longer. This is why you don't get chickenpox twice
Present from birth
Develops after exposure
Non-specific (attacks all)
Specific (targets one pathogen)
No memory
Has memory (stronger 2nd response)
Immediate response (0-12 hrs)
Delayed response (days-weeks)
Does NOT improve with exposure
Improves with each exposure
Neutrophils, macrophages, NK cells
B-cells, T-cells, antibodies
Active vs Passive Immunity
Active immunity involves the host producing its own antibodies (naturally by infection or artificially by vaccination). Passive immunity involves receiving pre-formed antibodies from another source (naturally via placenta/colostrum or artificially by injection).
Active and passive immunity describe HOW the body gets its antibodies. In active immunity, your body does the work — it recognizes the antigen and makes its own antibodies. In passive immunity, you receive ready-made antibodies from an external source.
Natural active — Getting infected and recovering (e.g., chickenpox → lifelong immunity)
Artificial active — Vaccination (e.g., polio vaccine, MMR, COVID vaccine). Weakened/killed pathogen or antigen fragment triggers immune response without causing disease
Natural passive — Antibodies from mother to baby. IgG crosses placenta (protects fetus). IgA in colostrum (first breast milk) protects newborn's gut
Artificial passive — Injection of pre-formed antibodies (antiserum/antitoxin). Example: anti-tetanus serum, anti-snake venom. Gives IMMEDIATE protection but no memory
Vaccination
Vaccines contain weakened/killed pathogens or their antigens that stimulate the immune system to produce memory cells without causing disease, providing long-term active immunity.
Vaccination is one of humanity's greatest inventions. Edward Jenner pioneered it in 1796 by inoculating a boy with cowpox to protect against smallpox — the word 'vaccine' comes from *vacca* (Latin for cow). Today, vaccines have eradicated smallpox and nearly eliminated polio.
Live attenuated vaccines — Weakened pathogen that can replicate but can't cause disease. Strong immunity, often single dose. Examples: OPV (oral polio), MMR (measles-mumps-rubella), BCG (tuberculosis)
Killed/Inactivated vaccines — Dead pathogen. Safer but weaker immunity, needs boosters. Examples: IPV (injectable polio), Rabies vaccine, Hepatitis A
Subunit/Recombinant vaccines — Only specific antigen, made using genetic engineering. Examples: Hepatitis B (recombinant surface antigen), HPV vaccine
Principle: All vaccines trigger primary immune response → memory B-cells and T-cells formed → on real infection, rapid secondary response neutralizes pathogen before disease develops
Immune System Components
The immune system has specialized organs and molecules. Lymphoid organs are either primary (thymus, bone marrow — where lymphocytes mature) or secondary (spleen, lymph nodes, MALT — where immune responses are initiated). Antibodies (immunoglobulins) are Y-shaped proteins with 5 classes: IgG, IgA, IgM, IgE, IgD.
Primary lymphoid organs are where lymphocytes are produced and mature. Bone marrow produces all blood cells and is where B-cells mature. Thymus is where T-cells mature.
Lymphocytes (B-cells and T-cells) are the soldiers of adaptive immunity. But before they can fight, they need training. Primary lymphoid organs are the training academies — they produce immature lymphocytes and turn them into competent immune cells that can recognize antigens.
Bone Marrow — Site of hematopoiesis (production of all blood cells including lymphocytes). B-cells both originate AND mature here. T-cell precursors are born here but migrate to thymus for maturation. 'B' in B-cell = Bone marrow (or Bursa of Fabricius in birds)
Thymus — Bilobed organ located behind the sternum (in the chest). T-cells migrate here from bone marrow and undergo maturation and selection. Only T-cells that recognize 'self' MHC but DON'T attack self-antigens survive. Thymus is large in children, shrinks (involutes) with age
Key function: Primary organs provide the microenvironment for lymphocyte maturation and self-tolerance education — so immune cells don't attack your own body
Secondary Lymphoid Organs
Secondary lymphoid organs are where mature lymphocytes encounter antigens and mount immune responses. They include the spleen, lymph nodes, tonsils, Peyer's patches, and MALT.
Once lymphocytes mature in primary organs, they migrate to secondary lymphoid organs where they wait for antigens. These organs are strategically placed throughout the body to intercept pathogens from blood, lymph, and mucosal surfaces.
Spleen — Largest secondary lymphoid organ. Filters blood (not lymph). Contains white pulp (lymphocytes) and red pulp (removes old/damaged RBCs). Acts as 'graveyard of RBCs'. Also a reservoir of blood
Lymph Nodes — Small bean-shaped organs along lymphatic vessels. Filter lymph (not blood). Trap antigens from lymph → present to lymphocytes → activate immune response. They swell during infections ('swollen glands')
MALT (Mucosa-Associated Lymphoid Tissue) — Lymphoid tissue along mucosal linings of respiratory, digestive, and urogenital tracts. Includes tonsils (throat), Peyer's patches (small intestine), and appendix. Constitutes about 50% of the body's lymphoid tissue
GALT (Gut-Associated Lymphoid Tissue) — Subset of MALT specific to the gut. Includes Peyer's patches and appendix. Critical because the gut is the largest entry point for pathogens
Antibodies (immunoglobulins) are Y-shaped glycoproteins made of 2 heavy chains + 2 light chains. The variable region (Fab) binds antigens; the constant region (Fc) determines the antibody class and effector function.
Antibodies are the key weapons of humoral immunity. Each antibody is a Y-shaped protein produced by plasma cells (activated B-cells). The 'arms' of the Y bind the antigen (like a lock and key), while the 'stem' signals immune cells to destroy the target.
Structure: 4 polypeptide chains — 2 identical heavy (H) chains + 2 identical light (L) chains, held together by disulfide bonds
Variable region (V) — Tips of the Y arms. Contains the antigen-binding site (paratope). Unique for each antibody — this is what gives specificity. Each antibody has 2 antigen-binding sites (bivalent)
Constant region (C) — Stem of the Y (Fc region). Same within a class. Determines antibody class (IgG/IgA/IgM/IgE/IgD) and biological function (complement activation, binding to phagocytes, crossing placenta)
Fab (Fragment antigen-binding) — The arms of the Y (variable + part of constant region). Fc (Fragment crystallizable) — The stem of the Y (constant region only)
Each human can produce an estimated 10 billion different antibody specificities — enough to recognize virtually any pathogen. This diversity comes from gene rearrangement (V-D-J recombination) in developing B-cells.
Five Classes of Antibodies
The five immunoglobulin classes differ in structure, location, and function. IgG is the most abundant, IgA protects mucosal surfaces, IgM is first responder, IgE mediates allergies, and IgD functions in B-cell activation.
IgG — Most abundant (75% of serum Ig). Monomer. Only Ig that crosses placenta (provides passive immunity to fetus). Activates complement. Main antibody in secondary response. Longest half-life (~23 days)
IgA — Second most abundant. Found as dimer in secretions (saliva, tears, breast milk/colostrum, mucus). 'Secretory antibody' — protects mucosal surfaces. Does NOT activate complement
IgM — Largest antibody (pentamer — 5 units joined). First antibody produced in primary immune response. Most efficient at complement activation (10 antigen-binding sites). Cannot cross placenta
IgE — Present in very low amounts in serum. Mediates allergic reactions — binds to mast cells and basophils. When allergen cross-links IgE on mast cells → degranulation → histamine release → allergy symptoms. Also fights parasitic worms
IgD — Found mainly on surface of mature B-cells. Acts as B-cell antigen receptor. Role in B-cell activation and differentiation. Present in very low amounts in serum
Most abundant in blood
IgG (75%)
Crosses placenta
IgG (only one)
First produced in infection
IgM
Largest antibody
IgM (pentamer)
In breast milk/colostrum
IgA
Mediates allergy
IgE
On B-cell surface
IgD
Allergies & Autoimmunity
Allergies are exaggerated immune responses to harmless substances (allergens) mediated by IgE and mast cells. Autoimmune diseases occur when the immune system attacks the body's own tissues (e.g., rheumatoid arthritis, SLE). Immunodeficiency means a weakened or absent immune response.
Allergies are hypersensitivity reactions where the immune system produces IgE antibodies against harmless environmental substances (allergens). Re-exposure triggers mast cell degranulation and histamine release.
An allergy is basically your immune system being a drama queen. It treats harmless substances — pollen, dust mites, peanuts, cat hair — as dangerous invaders and launches a full-scale attack. The result? Sneezing, itching, swelling, and in severe cases, life-threatening anaphylaxis.
First exposure (Sensitization) — Allergen enters body → B-cells produce IgE antibodies → IgE binds to surface of mast cells (in tissues) and basophils (in blood). No symptoms yet — this is the 'priming' phase
Autoimmune diseases occur when the immune system loses self-tolerance and produces antibodies or T-cells against the body's own cells and tissues. Examples include rheumatoid arthritis, SLE, Type 1 diabetes, and Graves' disease.
Normally, the immune system can distinguish 'self' from 'non-self' — this is called self-tolerance. It's established during lymphocyte development (T-cells in thymus, B-cells in bone marrow). When self-tolerance breaks down, the immune system attacks your own body — causing autoimmune disease.
Rheumatoid Arthritis (RA) — Immune system attacks the synovial membrane of joints → chronic inflammation → joint destruction, swelling, pain. Affects small joints (hands, feet) symmetrically
Systemic Lupus Erythematosus (SLE) — Antibodies against many self-antigens (DNA, histones, RBCs, platelets). Affects multiple organs — skin (butterfly rash on face), kidneys, joints, heart. More common in women
Type 1 Diabetes — T-cells destroy insulin-producing beta cells of pancreatic islets → no insulin production → blood glucose rises. Different from Type 2 (insulin resistance)
Graves' Disease — Antibodies stimulate TSH receptors on thyroid → overproduction of thyroid hormones → hyperthyroidism, bulging eyes (exophthalmos)
Myasthenia Gravis — Antibodies against acetylcholine receptors at neuromuscular junction → progressive muscle weakness
Immunodeficiency
Immunodeficiency means the immune system is weakened or absent. It can be primary (genetic — born with it) or secondary (acquired — caused by infection like HIV, malnutrition, or immunosuppressive drugs).
If allergy is the immune system overreacting and autoimmunity is it attacking self, immunodeficiency is the immune system being too weak to protect the body. Patients with immunodeficiency are susceptible to infections that healthy people easily fight off — these are called opportunistic infections.
Primary (Congenital) Immunodeficiency — Genetic defects present from birth. Examples: SCID (Severe Combined Immunodeficiency) — both B-cell and T-cell function absent. 'Bubble boy disease' — children must live in sterile environments
Secondary (Acquired) Immunodeficiency — Develops after birth due to external factors. AIDS (HIV destroys CD4+ T-cells) is the most important example. Others: immunosuppressive drugs (organ transplant patients), malnutrition, cancer chemotherapy, radiation
Opportunistic infections — Infections that only occur in immunocompromised people. Examples in AIDS: *Pneumocystis* pneumonia, *Toxoplasma* brain infection, Kaposi's sarcoma, oral thrush (*Candida*)
Overreaction to external harmless substance
Attack on body's own cells/tissues
Mediated by IgE + mast cells
Mediated by autoantibodies + autoreactive T-cells
Examples: hay fever, asthma, food allergy
Examples: RA, SLE, Type 1 diabetes
Treated with antihistamines
Treated with immunosuppressants
Target: allergen (external)
Target: self-antigens (internal)
AIDS
AIDS (Acquired Immuno Deficiency Syndrome) is caused by HIV (Human Immunodeficiency Virus), a retrovirus that destroys CD4+ helper T-cells. HIV replicates using reverse transcriptase, progressively weakening the immune system until opportunistic infections become fatal. Diagnosed by ELISA and confirmed by Western blot.
HIV RNA Reverse transcriptase DNA Integrase Host genome arrow New HIV particles
HIV Structure
HIV is an enveloped retrovirus with two copies of single-stranded RNA, reverse transcriptase, integrase, and protease enzymes. Its surface glycoproteins (gp120 and gp41) bind to CD4 receptors on helper T-cells.
HIV belongs to the retrovirus family — 'retro' because it reverses the normal flow of genetic information (RNA → DNA instead of DNA → RNA). It was first identified in 1983 and has since infected over 85 million people worldwide.
Genetic material: Two copies of single-stranded RNA (ss-RNA, positive sense). HIV is a diploid RNA virus
Reverse transcriptase (RT) — Converts viral RNA into DNA. This is the 'retro' step. Key drug target (AZT blocks RT)
Integrase — Inserts viral DNA into host cell's genome (becomes provirus). Once integrated, it's permanent
Protease — Cleaves viral polyproteins into functional proteins. Another drug target (protease inhibitors)
Envelope glycoproteins: gp120 (surface) binds to CD4 receptor on helper T-cells. gp41 (transmembrane) mediates membrane fusion for viral entry
Capsid: Made of p24 protein — used as a diagnostic marker (p24 antigen test)
HIV Replication Cycle
HIV binds to CD4+ T-cells via gp120, enters the cell, converts RNA to DNA using reverse transcriptase, integrates into the host genome, replicates using host machinery, and buds out to infect new cells.
HIV has a diabolically clever replication strategy. It specifically targets the cells that are supposed to coordinate the immune response against it — CD4+ helper T-cells. By destroying these 'generals' of immunity, HIV disables the entire immune army.
Step 1: Binding — gp120 on HIV binds to CD4 receptor on helper T-cell surface. Also needs a co-receptor (CCR5 or CXCR4)
Step 2: Fusion & Entry — gp41 mediates fusion of viral envelope with host cell membrane. Viral RNA + enzymes enter the cell
Step 4: Integration — Viral DNA enters nucleus → integrase inserts it into host chromosome → becomes provirus. Now it's a permanent part of the cell's DNA
Step 5: Transcription & Translation — Host cell machinery reads proviral DNA → makes viral mRNA → translates into viral proteins (using host ribosomes)
Step 6: Assembly & Budding — New viral RNA + proteins assemble at cell membrane → bud out in an envelope of host membrane → mature virions released. Host cell is destroyed
Step 7: Maturation — Protease cleaves polyproteins → functional virus ready to infect new CD4+ cells
Transmission, Progression & Diagnosis
HIV is transmitted through unprotected sex, contaminated blood/needles, and mother-to-child. It progresses through acute infection, clinical latency, and AIDS (when CD4 count drops below 200). Diagnosed by ELISA, confirmed by Western blot.
HIV is NOT transmitted through casual contact — no transmission from handshakes, hugs, sharing food, mosquito bites, or toilet seats. Understanding the actual routes of transmission is critical for both NEET and fighting the stigma around HIV/AIDS.
Sexual transmission — Most common route (unprotected vaginal, anal, or oral sex). Risk increases with multiple partners, presence of STDs, and lack of condom use
Blood-borne — Contaminated blood transfusion, sharing needles/syringes (IV drug users), needle-stick injuries in healthcare workers. Blood screening has largely eliminated transfusion risk
Mother-to-child (Vertical) — During pregnancy (transplacental), childbirth (contact with blood/fluids), or breastfeeding. Antiretroviral drugs during pregnancy reduce this risk to <1%
NOT transmitted by: Mosquitoes, casual contact, sharing utensils, swimming pools, tears, saliva (in normal amounts), sweat
Stage 1: Acute HIV infection (2-4 weeks) — Flu-like symptoms (fever, rash, sore throat). High viral load. Very infectious. Often misdiagnosed as flu
Stage 2: Clinical latency (Asymptomatic, 2-15 years) — Virus slowly destroys CD4+ cells. Person looks healthy but is infectious. CD4 count gradually drops
Stage 3: AIDS — CD4+ T-cell count drops below 200 cells/mm³ (normal: 500-1500). Severe immunodeficiency. Opportunistic infections (Pneumocystis pneumonia, Toxoplasmosis, Kaposi's sarcoma, Candidiasis) become fatal
ELISA (Enzyme-Linked Immunosorbent Assay) — Screening test. Detects anti-HIV antibodies in blood. Highly sensitive but can give false positives. If positive → confirmatory test needed
Western Blot — Confirmatory test. More specific than ELISA. Detects antibodies against specific HIV proteins (gp120, gp41, p24). If both ELISA + Western Blot positive → HIV confirmed
Prevention includes safe sexual practices, use of condoms, screening blood before transfusion, using disposable needles, avoiding sharing razors, and NACO (National AIDS Control Organization) awareness programs.
There is still no cure and no vaccine for HIV/AIDS. ART can control the virus but can't eliminate it (provirus hides in 'latent reservoirs'). Prevention remains the most effective strategy.
Safe sex — Use of condoms (latex/polyurethane) for every sexual encounter. Avoiding multiple sexual partners
Blood safety — Screening all donated blood for HIV (mandatory since 1985 in most countries). Avoiding unscreened blood transfusions
Needle safety — Using disposable/sterile needles and syringes. Never sharing needles (critical for IV drug users). Needle exchange programs
Mother-to-child prevention (PMTCT) — Antiretroviral drugs during pregnancy and delivery. Cesarean section if viral load is high. Formula feeding instead of breastfeeding (where safe water available)
NACO (National AIDS Control Organization) — India's nodal agency for HIV/AIDS prevention. Runs awareness campaigns, free ART centers, counseling, and testing
No vaccine available yet — HIV's extreme mutation rate and ability to hide in latent reservoirs have defeated all vaccine attempts so far
Cancer & Drug Abuse
Cancer is uncontrolled cell division caused by activation of oncogenes or inactivation of tumor suppressor genes. Tumors can be benign (localized) or malignant (metastatic). Drug and alcohol abuse involves opioids, cannabinoids, coca alkaloids, and their devastating effects on health and society.
Proto-oncogene mutation Oncogene arrow Uncontrolled cell division arrow Cancer
Cancer Fundamentals
Cancer is the uncontrolled proliferation of cells that have lost normal growth regulation. Normal cells become cancerous through genetic changes in proto-oncogenes and tumor suppressor genes.
Every second, millions of your cells are dividing in a tightly regulated manner. Cancer happens when this regulation breaks down — cells start dividing uncontrollably, ignore signals to stop, evade programmed death (apoptosis), and eventually invade other tissues. It's essentially cellular anarchy.
Normal cell properties: Contact inhibition (stop dividing when touching neighbors), respond to growth signals, undergo apoptosis when damaged, limited lifespan
Cancer cell properties: Loss of contact inhibition, ignore growth-stop signals, evade apoptosis, unlimited division (immortal), can invade other tissues
Neoplasm (tumor) — Any abnormal mass of tissue. Can be benign or malignant
Transformation — The process by which a normal cell becomes a cancer cell. Caused by mutations in growth-regulating genes
Localized, stays in one place
Invasive, spreads to distant sites
Slow growth
Rapid, uncontrolled growth
Encapsulated (has a capsule)
Not encapsulated, irregular borders
Does NOT metastasize
METASTASIZES (travels via blood/lymph)
Usually not life-threatening
Can be fatal
Example: Uterine fibroid, lipoma
Example: Carcinoma, sarcoma, leukemia
Metastasis & Types of Cancer
Metastasis is the spread of cancer cells from the primary tumor to distant organs via blood or lymph. Cancers are classified by tissue of origin: carcinomas (epithelial), sarcomas (connective), leukemias (blood), and lymphomas (lymphatic).
Metastasis is what makes cancer deadly. A tumor that stays in one place can usually be surgically removed. But when cancer cells break away, enter the blood or lymph, and colonize distant organs — that's metastasis, and it's extremely difficult to treat.
Metastasis steps: Cancer cells break loose from primary tumor → enter blood vessels or lymphatic vessels → travel to distant organs → exit vessels → establish secondary tumors. Common metastasis sites: lungs, liver, brain, bones
Carcinoma — Cancer of epithelial tissues. Most common type (~85% of all cancers). Examples: lung, breast, colon, skin, prostate cancer
Sarcoma — Cancer of connective tissues (bone, cartilage, muscle, fat). Examples: osteosarcoma (bone), chondrosarcoma (cartilage)
Leukemia — Cancer of blood-forming tissues (bone marrow). Abnormal WBCs flood the blood. No solid tumor. Types: ALL, AML, CLL, CML
Lymphoma — Cancer of lymphatic tissue. Examples: Hodgkin's lymphoma, Non-Hodgkin's lymphoma
Melanoma — Cancer of melanocytes (pigment cells in skin). Most dangerous skin cancer. Often caused by UV exposure
Oncogenes, Tumor Suppressors & Carcinogens
Cancer is caused by mutations in two types of genes: proto-oncogenes (become oncogenes when mutated — accelerate growth) and tumor suppressor genes (when inactivated — remove brakes on growth). Carcinogens are cancer-causing agents.
Think of cell division as a car. Proto-oncogenes are the gas pedal (promote growth), and tumor suppressor genes are the brakes (stop growth). Cancer happens when the gas pedal gets stuck ON (oncogene activation) or the brakes fail (tumor suppressor inactivation) — or both.
Proto-oncogenes — Normal genes that promote cell growth and division. When mutated, they become oncogenes — permanently 'on', driving uncontrolled cell division. Examples: ras (most commonly mutated oncogene in human cancers), myc, HER2
Tumor suppressor genes — Normal genes that put brakes on cell division and promote apoptosis. When inactivated (both copies must be lost), brakes are removed. Examples: p53 ('Guardian of the genome' — mutated in >50% of all cancers), Rb (retinoblastoma gene), BRCA1/BRCA2 (breast cancer)
Cancer is detected by biopsy, imaging (CT, MRI, PET), tumor markers, and molecular tests. Treatment includes surgery, radiation therapy, chemotherapy, and immunotherapy.
Early detection dramatically improves cancer survival rates. A cancer caught in Stage 1 (localized) has a much higher cure rate than Stage 4 (metastasized). This is why screening programs (mammograms, Pap smears, colonoscopies) are so important.
Biopsy — Gold standard for cancer diagnosis. A tissue sample is taken and examined microscopically by a pathologist. Histopathology confirms whether cells are cancerous
Imaging: CT scan, MRI, PET scan (detects metabolically active cancer cells using radioactive glucose), X-ray, ultrasound
Tumor markers — Substances (usually proteins) elevated in certain cancers. PSA (prostate cancer), CA-125 (ovarian cancer), AFP (liver cancer), CEA (colon cancer). Used for monitoring, not definitive diagnosis
Molecular tests — Detection of specific oncogenes/mutations using PCR, FISH, gene sequencing. Used for targeted therapy decisions
Treatment — Surgery: Physical removal of tumor. Best for localized cancers. Cure rate highest when caught early
Treatment — Radiation therapy: High-energy radiation kills cancer cells by damaging their DNA. Can be external beam or internal (brachytherapy)
Treatment — Chemotherapy: Drugs that kill rapidly dividing cells. Side effects: hair loss, nausea, immunosuppression (because it also kills normal fast-dividing cells like hair follicles, gut lining, bone marrow)
Treatment — Immunotherapy: Uses the patient's own immune system to fight cancer. Interferon-alpha boosts immune response against cancer cells. Monoclonal antibodies target specific cancer proteins
Drugs & Alcohol Abuse
Commonly abused substances include opioids (heroin, morphine), cannabinoids (marijuana), coca alkaloids (cocaine), and alcohol. They affect the CNS, cause dependence (psychological + physical), tolerance, and withdrawal symptoms.
Drug and alcohol abuse is a major health and social problem. These substances act on the brain's reward system, creating a cycle of dependence that's extremely difficult to break. NCERT covers this as part of human health because addiction directly impairs immunity, organ function, and mental health.
Opioids — Derived from opium poppy (*Papaver somniferum*). Bind to opioid receptors in brain → pain relief + euphoria. Examples: Morphine (medical painkiller), Heroin (diacetylmorphine — most dangerous, highly addictive, smoked/injected). Codeine (cough suppressant). Depressant effects: slows breathing, can cause fatal overdose
Cannabinoids — From *Cannabis sativa* (hemp plant). Active ingredient: THC (tetrahydrocannabinol). Forms: marijuana (dried flowers), hashish (resin), charas. Affects cardiovascular system, impairs memory, alters perception. Taken by inhalation or ingestion
Coca alkaloids — From *Erythroxylum coca*. Cocaine is a powerful CNS stimulant. Produces euphoria, increased energy, alertness. Extremely addictive. Taken by snorting, smoking (crack cocaine), or injection. Can cause cardiac arrest
Dependence — Compulsive need for the drug. Psychological dependence: craving, mental reliance. Physical dependence: body adapts to drug; removal causes withdrawal symptoms
Tolerance — Body needs increasing doses to achieve the same effect. Develops because receptors become less responsive
Withdrawal symptoms — Appear when drug is suddenly stopped. Include anxiety, tremors, nausea, sweating, pain, seizures (can be fatal for alcohol/benzodiazepines)
Addiction — Most severe form. Compulsive drug-seeking behavior despite harmful consequences. Changes brain circuits permanently
Prevention & Treatment of Drug Abuse
Prevention involves education, counseling, and strong social support. Treatment includes detoxification, rehabilitation, pharmacotherapy (methadone for opioid addiction), and psychotherapy. NCERT emphasizes the role of family, peers, and awareness.
Drug abuse prevention is better than cure — and NCERT emphasizes this strongly. The chapter highlights the role of parents, teachers, peers, and community in preventing adolescents from falling into substance abuse.
Why adolescents are vulnerable: Peer pressure (most common reason), curiosity and experimentation, need for excitement, coping with stress/failure, unstable family environment, easy availability of drugs
Prevention strategies: Education and awareness (school programs), building self-esteem and coping skills, strong family bonds and communication, avoiding peer pressure (saying NO), seeking professional help early
Treatment — Detoxification: Medically supervised withdrawal. Drugs are gradually reduced while managing withdrawal symptoms
Treatment — Rehabilitation: Long-term programs to prevent relapse. Includes counseling, behavioral therapy, support groups (like Narcotics Anonymous)