Health Disease

Health Disease

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Health, by the WHO (1948), is a state of complete physical, mental, and social well-being — not merely the absence of disease. A disease is any disturbance in structure or function impairing normal physiology, while a disorder is a functional abnormality without a known structural cause. Infectious diseases are caused by pathogens (bacteria, viruses, fungi, protozoans, helminths) and spread via vectors, fomites, body fluids, or air. Non-infectious diseases arise from genetic, nutritional, lifestyle, or environmental factors. NEET high-yield facts: malaria is transmitted only by the female Anopheles mosquito (Plasmodium’s definitive host), Wuchereria bancrofti (transmitted by Culex) causes filariasis/elephantiasis, and Salmonella typhi (Widal test) causes typhoid. HIV is a retrovirus attacking helper T cells (CD4⁺). Antibiotics work only on bacteria, never on viruses. Endemic = constant in a region; epidemic = sudden outbreak; pandemic = global spread.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Categories of Disease

Infectious diseases stem from pathogens — bacteria (release toxins, e.g., Salmonella typhi), viruses (oblige living cells, e.g., HIV, rhinovirus), fungi (Candida), protozoans (Plasmodium, Entamoeba histolytica), and helminths (Ascaris, Wuchereria). Non-infectious causes include genetic mutations (sickle-cell anaemia), nutritional deficiencies (goitre from iodine lack), and lifestyle factors (smoking → lung cancer, alcohol → cirrhosis).

Modes of Transmission

Transmission may be direct contact (skin-to-skin), indirect contact through fomites (contaminated objects), airborne droplet infection (common cold, tuberculosis), vector-borne (mosquitoes transmitting Plasmodium and Wuchereria), and exchange of body fluids (HIV, hepatitis B). A reservoir is the long-term host harbouring the pathogen; humans are reservoirs for typhoid, while animals serve as reservoirs for zoonoses like rabies.

Immunology Basics

Innate immunity provides non-specific barriers (skin, mucus, phagocytes). Adaptive immunity involves B lymphocytes (produce antibodies IgG, IgM, IgA, IgE) and T lymphocytes (helper CD4⁺ and cytotoxic CD8⁺ cells). The primary immune response is slow and produces memory cells; the secondary response is faster and stronger due to memory. Active immunity arises from infection or vaccination; passive immunity is transferred (maternal antibodies, antiserum).

Key Diseases for NEET

Disease	Pathogen	Vector / Route	Diagnostic
Malaria	Plasmodium vivax/falciparum	Female Anopheles	Blood smear
Filariasis	Wuchereria bancrofti	Culex mosquito	Blood smear (night)
Typhoid	Salmonella typhi	Contaminated food/water	Widal test
AIDS	HIV (retrovirus)	Body fluids	ELISA
Amoebiasis	Entamoeba histolytica	Faecal–oral	Stool microscopy

Prevention

Vaccines (smallpox eradicated, polio, DPT, BCG, hepatitis B) induce active immunity. Antibiotics target bacteria only — they are useless against the common cold (rhinovirus) and AIDS. Vector control, sanitation, and safe drinking water break transmission chains.

Common Mistakes

• Conflating disease (known cause) with disorder (no identifiable cause).
• Assuming any mosquito transmits malaria — only female Anopheles does, because she requires a blood meal for egg development.
• Prescribing antibiotics for viral fevers.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge Cases and Subtleties

The term zoonosis describes diseases jumping from vertebrate animals to humans (rabies, plague, Nipah). Opportunistic infections (e.g., Candida, Pneumocystis) exploit weakened immunity, as in late-stage HIV when CD4⁺ counts collapse. Oncogenic viruses like HPV and EBV trigger cancers by inserting oncogenes or disabling tumour-suppressor genes (p53, Rb), allowing uncontrolled proliferation and metastasis. Drug and alcohol abuse depress the CNS (cerebellum → ataxia) and cause hepatic cirrhosis through sustained oxidative damage.

Worked Micro-Example

A patient presents with cyclical fever with chills every 48–72 hours and splenomegaly. Microscopy of a Giemsa-stained blood smear shows ring-shaped trophozoites inside RBCs. Diagnosis: malaria caused by Plasmodium vivax. The pathogen’s definitive host is the female Anopheles mosquito, where sexual reproduction (sporogony) occurs; humans are intermediate hosts undergoing asexual schizogony. Treatment uses chloroquine (vivax) or artemisinin-based combination therapy. Prevention relies on insecticide-treated nets and eliminating stagnant water — larval breeding sites.

Connections to Adjacent Topics

• Genetics: cancer links directly to mutations in proto-oncogenes and tumour suppressors covered in Molecular Basis of Inheritance.
• Evolution: antibiotic resistance in Mycobacterium and Staphylococcus is a textbook case of natural selection.
• Biotechnology: ELISA and PCR for pathogen detection derive from recombinant DNA techniques.

Practice Prompts

1. Why is ELISA preferred over the Widal test for HIV screening, and what principle does ELISA exploit?
2. A child with dog-bite history shows hydrophobia. Identify the pathogen class, reservoir, and post-exposure prophylaxis regimen (vaccine + immunoglobulin).

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