Human health and disease is one of the highest-yield chapters in NEET Biology, consistently contributing 3–4 questions per year. It spans infectious diseases caused by bacteria, viruses, protozoans, and helminths; the immune system; cancer biology; and substance abuse. Mastery of pathogen–vector–diagnostic test associations is the single most critical skill for this chapter.
Bacterial Diseases. Typhoid is caused by Salmonella typhi, which enters through contaminated food and water and colonises the small intestine. The hallmark is a sustained high fever and, in severe cases, intestinal perforation. The Widal test detects antibodies against S. typhi and is the standard confirmatory method. Pneumonia is caused by Streptococcus pneumoniae and Haemophilus influenzae. The pathogens fill alveoli with fluid, severely impairing gas exchange and causing chest pain and breathing difficulty.
Viral Diseases. The common cold is caused by Rhinovirus and primarily affects the upper respiratory tract. AIDS is caused by HIV, a retrovirus. HIV carries reverse transcriptase — an enzyme that converts the viral single-stranded RNA genome into double-stranded DNA, which then integrates into the host's chromosomes. HIV specifically and selectively infects CD4+ T-helper lymphocytes, progressively destroying the cellular arm of adaptive immunity. Without T-helper cells, the body loses coordination of both humoral and cell-mediated responses, leading to opportunistic infections. ELISA is the screening test; Western blot is confirmatory. Transmission routes are sexual contact, contaminated blood/needles, and mother-to-child (vertical transmission).
Protozoan Diseases. Malaria is caused by four Plasmodium species, of which P. vivax (benign tertian malaria, 48-hour fever cycle) and P. falciparum (malignant tertian malaria, most dangerous) are NEET favourites. The vector is exclusively the female Anopheles mosquito. The parasite's life cycle includes an asexual phase in the human liver (liver schizogony, where sporozoites multiply into merozoites) and an erythrocytic phase (RBC schizogony, where merozoites rupture RBCs, releasing haemozoin pigment — this pigment causes the characteristic cyclic fever and chills). Sexual gametocytes formed in human blood are taken up by the mosquito, completing the cycle. Amoebiasis (amoebic dysentery) is caused by Entamoeba histolytica, which infects the large intestine through contaminated food/water, causing abdominal pain and bloody diarrhoea.
Helminth Diseases. Ascariasis is caused by Ascaris lumbricoides, a large intestinal worm transmitted through contaminated soil/food. It causes intestinal obstruction and muscular pain. Filariasis (elephantiasis) is caused by Wuchereria bancrofti or W. malayi. The worms lodge in the lymphatic vessels of the lower limbs, causing chronic inflammation, blockage, and massive swelling (lymphoedema). The vector is the Culex mosquito — a critical distinction from Anopheles (malaria).
Immunity. The immune system operates at two levels. Innate (non-specific) immunity includes four barrier types: physical barriers (skin, mucous membranes), physiological barriers (stomach acid, lysozyme in tears/saliva), cellular barriers (NK cells, neutrophils, macrophages performing phagocytosis), and cytokine barriers (interferons that protect surrounding cells during viral infection). Adaptive (specific) immunity develops after antigen exposure. Humoral immunity: B-lymphocytes differentiate into plasma cells that secrete antibodies. The five immunoglobulin classes are IgA (secretory, in colostrum/saliva), IgD (B-cell receptor), IgE (allergy, anti-parasitic), IgG (most abundant, crosses placenta), and IgM (first antibody in primary response). Cell-mediated immunity: T-lymphocytes include CD4+ T-helper cells (coordinate immune response), CD8+ cytotoxic T-cells , and suppressor T-cells (regulate immune response).
Active immunity is when the individual's own immune system generates antibodies — either naturally after infection or artificially through vaccination. It is slow to develop (days to weeks) but long-lasting, because memory B and T cells are formed. Passive immunity involves receiving preformed antibodies from an external source — naturally via IgG crossing the placenta or IgA in colostrum, or artificially via antiserum/immunoglobulin injection. It acts immediately but is short-lived because no memory cells are formed.
Allergies represent exaggerated immune responses: allergens trigger IgE-coated mast cells to degranulate and release histamine, causing inflammation, sneezing, and bronchoconstriction. Antihistamines block these effects. Autoimmunity (e.g., rheumatoid arthritis, SLE) occurs when immune cells attack self-antigens due to failure of self-tolerance.
Cancer. Normal cells have regulated division controlled by proto-oncogenes and tumour suppressor genes. Carcinogens (physical: UV radiation, ionising radiation; chemical: tobacco carcinogens, benzene; biological: oncogenic viruses like HPV, EBV) can transform normal cells into cancer cells. Activated proto-oncogenes become oncogenes that drive uncontrolled proliferation. Tumour suppressor genes (p53 — "guardian of genome"; Rb) normally halt the cell cycle; their loss permits tumour growth. Benign tumours are encapsulated, slow-growing, and non-invasive. Malignant tumours invade surrounding tissue and metastasise via blood or lymph to distant organs. Detection uses biopsy (gold standard), CT/MRI/PET scanning, and molecular markers. Treatment includes surgery, radiation therapy, chemotherapy, and immunotherapy (including monoclonal antibodies).
Substance Abuse. Heroin (diacetylmorphine), an opioid, is chemically synthesised from morphine extracted from Papaver somniferum (opium poppy). It depresses the CNS and is highly addictive. Cannabinoids (marijuana, hashish, ganja) come from Cannabis sativa; they affect the cardiovascular system and alter perception. Cocaine, extracted from Erythroxylum coca, is a CNS stimulant that blocks dopamine reuptake in the brain's reward pathway, causing euphoria and addiction. Nicotine in tobacco is addictive and carcinogenic, with links to lung cancer, emphysema, and cardiovascular disease.