Human Health & Disease

Human health and disease is among the highest-yield topics in Biology for NEET, covering infectious diseases, immunity, cancer, and substance abuse. Understanding pathogen-disease-vector associations and the immune response framework is critical.

Bacterial diseases include typhoid, caused by Salmonella typhi, which colonizes the small intestine leading to intestinal perforation and sustained high fever. The Widal test confirms diagnosis. Pneumonia, caused by Streptococcus pneumoniae and Haemophilus influenzae, fills alveoli with fluid, impairing gas exchange.

Viral diseases range from the common cold (Rhinovirus, affecting the upper respiratory tract) to AIDS. HIV is a retrovirus that specifically attacks CD₄+ T-helper cells. It carries reverse transcriptase, which converts viral RNA into DNA that integrates into the host genome. Transmission occurs through sexual contact, contaminated blood, shared needles, and from mother to child. ELISA is the standard screening test.

Protozoan diseases are NEET favorites. Malaria is caused by Plasmodium species — P. vivax (benign tertian, 48-hour fever cycle) and P. falciparum (malignant tertian). The vector is the female Anopheles mosquito. The parasite undergoes liver schizogony, then RBC schizogony (releasing haemozoin pigment), before forming gametocytes taken up by the mosquito. Amoebiasis (amoebic dysentery) is caused by Entamoeba histolytica, transmitted through contaminated food and water, affecting the large intestine.

Helminth diseases include ascariasis (Ascaris lumbricoides — intestinal blockage, muscular pain) and filariasis or elephantiasis (Wuchereria bancrofti/W. malayi — chronic inflammation and swelling of lymphatic vessels in lower limbs; vector is the Culex mosquito, not Anopheles).

Immunity operates at two levels. Innate immunity is non-specific, comprising physical barriers (skin, mucous membranes), physiological barriers (stomach acid, lysozyme in tears), cellular barriers (NK cells, neutrophils, macrophages performing phagocytosis), and cytokine barriers (interferons). Adaptive immunity is specific: humoral immunity involves B-lymphocytes differentiating into plasma cells that produce antibodies (IgA, IgD, IgE, IgG, IgM), while cell-mediated immunity uses T-lymphocytes (helper, cytotoxic, suppressor). Active immunity develops when the body produces its own antibodies — naturally after infection or artificially through vaccination. It is slow-onset but long-lasting with memory cells. Passive immunity uses preformed antibodies — naturally via maternal IgG across the placenta or IgA in colostrum, or artificially through antiserum injection. It acts immediately but is short-lived with no memory.

Allergies result from exaggerated IgE-mediated responses: allergens trigger mast cells to release histamine, causing inflammation. Autoimmunity occurs when the immune system attacks self-tissues (e.g., rheumatoid arthritis).

Cancer involves transformation of normal cells by physical (UV, radiation), chemical (tobacco carcinogens), or biological agents (oncogenic viruses). Oncogenes (activated proto-oncogenes) drive uncontrolled division, while tumor suppressor genes (p53, Rb) normally inhibit it. Benign tumors remain localized; malignant tumors undergo metastasis, spreading via blood and lymph. Detection methods include biopsy, CT, MRI, and molecular markers. Treatment involves surgery, radiation, chemotherapy, and immunotherapy.

Drug abuse is tested factually. Opioids such as heroin (diacetylmorphine, derived from morphine from Papaver somniferum) depress the CNS. Cannabinoids from Cannabis sativa affect the cardiovascular system. Cocaine from Erythroxylum coca stimulates the CNS by interfering with dopamine transport. Tobacco (nicotine) causes lung cancer and emphysema.

The key testable concept is the pathogen-vector-diagnostic test association for major infectious diseases, particularly that HIV targets CD₄+ T-helper cells, Anopheles transmits malaria, and Culex transmits filariasis.