Why Do We Fall Ill

Disease is both a scientific and a policy challenge. UPSC GS2 (Health) and GS3 (Science & Technology) regularly test concepts from this chapter — the difference between communicable and non-communicable diseases, how epidemics spread, why vaccines work, and how India's health infrastructure responds to disease outbreaks. The COVID-19 pandemic, antimicrobial resistance, zoonotic diseases, and the One Health approach all have their foundations in this chapter.

PART 1 — Quick Reference Tables

Health vs Disease

Types of Diseases

Pathogens and Examples

Disease Transmission Routes

PART 2 — Detailed Notes

1. Defining Health

The World Health Organization (WHO) defines health as "a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity." This holistic definition has important policy implications:

• Mental health is integral to health — India's Mental Healthcare Act 2017 recognises the right to mental health.
• Social determinants of health (income, education, sanitation, nutrition) determine disease burden more than individual behaviour — the basis of welfare schemes like POSHAN Abhiyaan, Swachh Bharat Mission, and Jal Jeevan Mission.
• Universal Health Coverage (UHC) — SDG 3.8 — aims for health services for all. India's PM-Ayushman Bharat is a step toward UHC.

Community health is distinct from individual health. A disease-free individual living in a community without safe water, sanitation, or economic security cannot maintain health — hence public health infrastructure matters.

2. Causes of Disease

Diseases have immediate causes (the pathogen or immediate trigger) and contributory causes (malnutrition, lack of clean water, overcrowding, poverty — which create conditions for disease to take hold).

Immediate cause alone is not enough — Koch's postulates established how to prove a specific microorganism causes a specific disease (isolate the pathogen; reproduce disease in healthy host; re-isolate same pathogen). But poverty and sanitation determine whether the immediate cause reaches individuals and whether they can recover.

3. Infectious Diseases — Pathogens in Detail

Bacterial diseases:

• Tuberculosis (TB): Mycobacterium tuberculosis. Air-borne. Primarily infects lungs (pulmonary TB) but can affect any organ. India has the highest TB burden globally — about 2.8 million new cases per year. Revised National TB Control Programme (RNTCP) / now National TB Elimination Programme (NTEP). India's 2025 elimination target was not met (incidence ~187/lakh vs. <10/lakh threshold; WHO Global TB Report 2025); India aligns with the global 2030 elimination target. [Additional] WHO Global TB Report 2025: India's TB incidence fell to 187 per lakh (2024), down 21% from 237 per lakh in 2015 — nearly double the global rate of 12% decline. TB deaths: 21 per 100,000 (2024). Treatment coverage: 92%. India now targets 80% incidence reduction and 90% deaths reduction by 2030 vs 2015 baseline (aligned with global End TB Strategy).
• Cholera: Vibrio cholerae. Water-borne. Severe watery diarrhoea can lead to death by dehydration within hours. Oral Rehydration Salts (ORS) — a simple, cheap cure — is one of public health's greatest success stories.
• Typhoid: Salmonella typhi. Food and water-borne. Typhoid conjugate vaccine (TCV) introduced in India's Universal Immunisation Programme.

Viral diseases:

• COVID-19: Caused by SARS-CoV-2 (a coronavirus). Air-borne via aerosols. Virus uses ACE2 receptors on human cells. India developed COVAXIN (inactivated virus vaccine by Bharat Biotech) and administered COVISHIELD (Oxford-AstraZeneca adenoviral vector vaccine). India's CoWIN platform was praised globally for vaccine delivery management.
• HIV/AIDS: Human Immunodeficiency Virus attacks CD4+ T-helper cells, destroying the immune system. Transmitted sexually, through blood (shared needles), and vertically (mother to child). India's NACO (National AIDS Control Organisation) runs the National AIDS Control Programme. Antiretroviral therapy (ART) controls but does not cure HIV.
• Dengue: Flavivirus transmitted by Aedes aegypti mosquito. No specific antiviral; treatment is symptomatic. Four serotypes — infection with one does not provide cross-protection for others (secondary infection with different serotype causes severe dengue).

Protozoan diseases:

• Malaria: Plasmodium species (P. falciparum — most dangerous; P. vivax — most common in India). Transmitted by female Anopheles mosquito. Parasite life cycle alternates between mosquito (sexual stage) and human liver/blood (asexual stage). India's National Vector Borne Disease Control Programme (NVBDCP) manages malaria control. Drug resistance in P. falciparum (to chloroquine) is a major challenge.
• Kala-azar (Visceral Leishmaniasis): Leishmania donovani. Transmitted by sandfly. Affects liver, spleen, bone marrow. Targeted for elimination in India.

4. The Immune System

The body has multiple lines of defence:

First line (non-specific barriers):

• Skin — physical barrier; sweat and sebum (acidic, antibacterial)
• Mucus membranes lining respiratory, digestive, and urinary tracts
• Cilia in respiratory tract — sweep pathogens upward (mucociliary escalator)
• Stomach acid (pH ~1.5–2) — kills most ingested bacteria

Second line (innate/non-specific immune response):

• Inflammation — increased blood flow, recruitment of phagocytes
• Phagocytosis — neutrophils and macrophages engulf and destroy pathogens
• Natural killer cells — destroy virus-infected cells
• Fever — elevated temperature inhibits bacterial growth; speeds up immune cell activity

Third line (adaptive/specific immune response):

• B-lymphocytes — produce antibodies (immunoglobulins) specific to antigen
• T-lymphocytes — cytotoxic T-cells destroy infected cells; helper T-cells coordinate immune response
• Immunological memory — after first exposure, memory B and T cells remain; second exposure triggers rapid, amplified response — the basis of vaccination

💡 Explainer: Antigens and Antibodies

An antigen is any molecule (usually a protein on the surface of a pathogen) that triggers an immune response. An antibody is a Y-shaped protein (immunoglobulin) produced by B-cells that binds specifically to its antigen — like a lock and key. Antibody-antigen binding marks the pathogen for destruction by phagocytes.

Antibodies have four functions: neutralisation (block pathogen entry into cells), opsonisation (coat pathogen so phagocytes recognise it), complement activation (trigger a cascade that destroys the pathogen), and agglutination (clump pathogens together).

5. Vaccination

Edward Jenner (1796) — demonstrated that cowpox infection protected against smallpox; this was the first vaccine. Louis Pasteur — developed vaccines for cholera, anthrax, and rabies; coined the term "vaccine" in honour of Jenner (from Latin vacca = cow). Robert Koch — developed techniques to isolate and grow bacteria; established Koch's postulates for proving disease causation.

Types of vaccines:

• Live attenuated: Weakened (attenuated) but living pathogen. Strong, long-lasting immunity. Examples: BCG (TB), OPV (oral polio), MMR (measles-mumps-rubella).
• Inactivated (killed): Killed pathogen. Safer but requires booster doses. Examples: IPV (injectable polio), Hepatitis A, COVAXIN.
• Subunit/recombinant: Uses only specific antigens (proteins) of the pathogen. Examples: Hepatitis B, HPV vaccine (Cervarix, Gardasil).
• mRNA vaccines: Deliver mRNA coding for the antigen; cells produce the protein, triggering immunity. Examples: Pfizer-BioNTech, Moderna (COVID-19). No live pathogen; cannot cause disease.
• Viral vector vaccines: Harmless virus delivers antigen gene. Examples: COVISHIELD (Oxford-AstraZeneca), Sputnik V.
• Toxoid vaccines: Inactivated bacterial toxins. Examples: Tetanus, Diphtheria.

5a. [Additional] Herd Immunity and R₀ — The Mathematics of Epidemics

Basic Reproduction Number (R₀): The average number of people one infected person will infect in a completely susceptible population. If R₀ > 1, the epidemic grows; if R₀ < 1, it dies out.

Herd immunity: When enough of a population is immune (through vaccination or prior infection), the chain of transmission breaks — even unvaccinated individuals are protected because the pathogen cannot find new hosts.

Herd immunity threshold (HIT) = 1 − 1/R₀

For measles (R₀ = 15): HIT = 1 − 1/15 = 93.3% — 93.3% of the population must be immune For COVID-19 (R₀ = 3): HIT = 1 − 1/3 = 66.7%

6. Principles of Treatment vs Prevention

Treatment addresses disease after it occurs: antibiotics for bacterial infections, antivirals for some viral infections, antipyretics for fever, fluid replacement for dehydration.

Prevention is more effective and cheaper than treatment:

• Primary prevention: Prevent disease before it occurs — vaccination, sanitation, vector control, safe water.
• Secondary prevention: Early detection and treatment — screening programmes, DOTS for TB.
• Tertiary prevention: Limit disability and complications in those already affected — rehabilitation.

🎯 UPSC Connect: Antimicrobial Resistance (AMR)

AMR is the ability of microorganisms to withstand drugs that once killed them. It is a Global Health Emergency:

• Over 700,000 deaths annually from AMR globally; projected to reach 10 million by 2050 (O'Neill Commission Report, 2016).
• India is one of the highest consumers of antibiotics globally — contributing to resistance.
• India's NAP-AMR 2017-2021 (first plan) had limited state-level implementation — only Kerala, MP, Delhi, AP, Gujarat, Sikkim, Punjab developed state action plans. [Additional] NAP-AMR 2.0 (2025-2029) launched by Union Health Minister on November 18, 2025 — five-year plan with six pillars: (i) awareness and education, (ii) surveillance and laboratory capacity, (iii) infection prevention and control, (iv) antimicrobial stewardship, (v) research and innovation, (vi) governance and coordination. Key addition: private sector engagement elevated as a priority (private sector delivers majority of healthcare and veterinary services in India).
• ESKAPE pathogens — six bacteria most prone to AMR — are priority pathogens for WHO.
• AMR makes common surgeries, cancer treatment, and organ transplants risky.

🎯 UPSC Connect: One Health Approach

The One Health framework recognises that human health, animal health, and environmental health are inextricably linked. About 60% of known infectious diseases are zoonotic (originate in animals); 75% of new or emerging diseases are zoonotic (WHO).

Examples: COVID-19 (likely bat origin), Nipah virus (fruit bats), Bird flu H5N1 (poultry), Rabies (dogs/bats), Brucellosis (cattle).

India's One Health Consortium (ICAR-DBT-DST-ICMR collaboration) integrates human, animal, and environmental health surveillance.

PART 3 — Frameworks & Analysis

Framework: Disease Burden — Epidemiological Transition

India is experiencing a double burden of disease — still fighting infectious diseases (TB, malaria, dengue) while facing a rising tide of non-communicable diseases (diabetes, cardiovascular disease, cancer, mental health disorders):

Framework: India's Health Programmes (Disease-wise)

Exam Strategy

Prelims traps:

• Dengue is caused by a virus (Flavivirus), transmitted by Aedes mosquito — NOT Anopheles (which transmits malaria).
• Plasmodium is a protozoan, not a bacterium.
• Kala-azar is caused by Leishmania — transmitted by sandfly, not mosquito.
• Antibiotics work against bacteria, NOT viruses — using antibiotics for viral infections is a driver of AMR.
• COVID-19 pathogen is SARS-CoV-2 (a coronavirus) — not SARS-CoV-1 (responsible for 2002-03 SARS outbreak).

Mains (GS2/GS3) frameworks:

• AMR: science → health consequence → economic cost → India's NAP-AMR → global governance (WHO Global Action Plan on AMR)
• One Health: COVID-19/Nipah as case studies → animal-human-environment nexus → India's policy response
• Universal Health Coverage: WHO definition of health → NHM → Ayushman Bharat → gaps in coverage

Practice Questions

Q1 (Prelims 2023): Consider the following diseases: Dengue, Malaria, Typhoid, Chikungunya. Which are transmitted by mosquitoes? (Tests vector-disease pairing — a core MCQ area)

Q2 (Prelims 2021): With reference to Antimicrobial Resistance, what is the difference between "resistance" and "tolerance" in bacteria? (Tests understanding of AMR mechanisms — rooted in pathogen biology)

Q3 (Mains GS3 2020): COVID-19 pandemic has highlighted the need for a One Health approach. Elaborate. (Direct application of pathogen classification → zoonosis → One Health)

Q4 (Mains GS2 2018): Appropriate local community-level healthcare intervention is a prerequisite to achieve 'Health for All' in India. Explain. (Connects community health concept from this chapter to UHC policy)