Human Health and Disease
How pathogens cause common human diseases and how the body's innate and acquired immune systems defend against them, framed by health as complete physical, mental and social well-being.
Prelims regularly tests disease-pathogen-vector matching, the Widal test, types of immunity and antibody classes as direct Science & Tech factual questions. For Mains it feeds GS-III (science in everyday life, biotech vaccines, disease-control technology) and overlaps GS-II health-as-social-sector themes like immunisation and public-health policy.
Understand the chapter
What 'Health' Really Means
Health is defined as a state of complete physical, mental and social well-being, not merely the absence of disease or physical fitness (the WHO formulation NCERT adopts). Early Greeks (Hippocrates) and Indian Ayurveda explained health as a balance of body 'humors', an idea disproved when William Harvey experimentally demonstrated blood circulation and the thermometer showed normal temperature in 'blackbile' persons. Modern biology links mind, via neural and endocrine systems, to the immune system and thus to health.
- Determinants of health: genetic disorders (inherited), infections, and lifestyle (food, water, rest, exercise, habits).
- Good health raises productivity and longevity and lowers infant and maternal mortality.
- Maintained by balanced diet, hygiene, exercise, yoga, vaccination, waste disposal and vector control.
Pathogens - The Disease-Causing Agents
Diseases are broadly infectious (easily transmitted person-to-person) or non-infectious (e.g., cancer). Any disease-causing organism is a pathogen; most parasites are pathogens because they harm the host they live in or on. Pathogens enter, multiply and disrupt vital activities, and must adapt to the host environment, such as surviving the stomach's low pH.
- Five pathogen groups: bacteria, viruses, fungi, protozoans, helminths.
- Pathogen damage is both morphological and functional.
- Among infectious diseases AIDS is fatal; among non-infectious diseases cancer is the major killer.
Bacterial and Viral Diseases
Salmonella typhi causes typhoid via contaminated food and water reaching the small intestine, with sustained 39-40 C fever, confirmed by the Widal test. Streptococcus pneumoniae and Haemophilus influenzae cause pneumonia, flooding the alveoli with fluid and impairing respiration. Rhino viruses cause the common cold, infecting the nose and respiratory passage but not the lungs, lasting 3-7 days.
- Typhoid: severe cases cause intestinal perforation; carrier 'Typhoid Mary' (Mary Mallon) spread it for years.
- Pneumonia: severe cases show gray-bluish lips and nails; spreads by droplets and shared utensils.
- Other bacterial diseases: dysentery, plague, diphtheria.
Protozoan, Helminthic and Fungal Diseases
Malaria is caused by Plasmodium (notably P. vivax and the fatal, malignant P. falciparum), transmitted by the female Anopheles mosquito, the vector. Rupture of infected RBCs releases the toxin haemozoin, causing chills and fever recurring every 3-4 days, and the parasite needs two hosts. Entamoeba histolytica (amoebiasis), Ascaris (ascariasis), Wuchereria (filariasis/elephantiasis) and dermatophytic fungi (ringworm) round out the common diseases.
- Malaria cycle: sporozoites (infectious form) infect liver cells, then RBCs; human + mosquito are the two hosts.
- Amoebiasis: Entamoeba histolytica in the large intestine; houseflies are mechanical carriers.
- Filariasis/elephantiasis: Wuchereria bancrofti and W. malayi in lymphatic vessels of the lower limbs.
- Ringworm: fungi Microsporum, Trichophyton, Epidermophyton cause dry, scaly, itchy lesions on skin, nails, scalp.
Prevention, Vectors and the Vaccine Triumph
Personal and public hygiene break food- and water-borne transmission (typhoid, amoebiasis, ascariasis), while air-borne diseases (pneumonia, common cold) also need avoidance of close contact. Vector-borne diseases (malaria, filariasis) demand elimination of mosquito breeding sites. Vaccination has eradicated smallpox and controlled polio, diphtheria, pneumonia and tetanus, with biotechnology promising newer, safer vaccines.
- Vector control: remove stagnant water, clean coolers, use nets and wire-mesh, spray insecticides, introduce Gambusia fish that eat mosquito larvae.
- Aedes mosquitoes spread dengue and chikungunya, seen in recent Indian outbreaks.
- Smallpox was completely eradicated by immunisation; antibiotics treat many infections.
Immunity I - Innate (Non-specific) Defence
Immunity is the host's overall ability to fight disease-causing organisms, conferred by the immune system. Innate immunity is non-specific and present from birth, working through four barrier types that block or destroy invaders regardless of identity.
- Physical: skin; mucus lining of respiratory, gastrointestinal and urogenital tracts.
- Physiological: stomach acid, saliva, tears.
- Cellular: neutrophils (PMNL), monocytes, natural killer cells and macrophages phagocytose microbes.
- Cytokine: interferons from virus-infected cells protect neighbouring uninfected cells.
Immunity II - Acquired (Specific) Defence
Acquired immunity is pathogen-specific and shows memory: the first encounter gives a weak primary response, later encounters a strong secondary (anamnestic) response. B-lymphocytes secrete antibodies (humoral, antibody-mediated immunity), while T-lymphocytes mediate cell-mediated immunity (CMI) and drive graft rejection by distinguishing 'self' from 'nonself'. On antigen exposure the host makes its own antibodies (active immunity), whereas ready-made antibodies given to the body confer passive immunity.
- Antibody structure: H2L2 - two heavy and two light peptide chains; classes IgA, IgM, IgE, IgG.
- Humoral (B-cell, antibody) versus cell-mediated (T-cell) immunity.
- Transplants need tissue and blood-group matching plus lifelong immunosuppressants.
Key terms
- Pathogen
- Any disease-causing organism (bacterium, virus, fungus, protozoan, helminth) that harms its host.
- Vector
- A transmitting agent that carries a pathogen between hosts, e.g., female Anopheles mosquito for malaria.
- Innate immunity
- Non-specific defence present from birth via physical, physiological, cellular and cytokine barriers.
- Acquired immunity
- Pathogen-specific defence with memory, mediated by B- and T-lymphocytes.
- Antibody
- B-cell protein (H2L2) that binds pathogens; classes include IgA, IgM, IgE, IgG.
- Humoral immunity
- Antibody-mediated response carried in the blood by B-lymphocytes.
- Cell-mediated immunity (CMI)
- T-lymphocyte response responsible for graft rejection and self/nonself discrimination.
- Interferon
- Protein secreted by virus-infected cells that protects neighbouring uninfected cells.
- Anamnestic response
- The intensified secondary immune response on re-exposure to the same pathogen.
- Sporozoite
- The infectious stage of Plasmodium injected by the female Anopheles mosquito.
Must-know facts exam-ready
- Health = complete physical, mental and social well-being, not merely absence of disease (WHO definition adopted by NCERT).
- William Harvey experimentally demonstrated blood circulation, helping disprove the 'humors' theory of health.
- Typhoid: Salmonella typhi; confirmed by the Widal test; sustained fever 39-40 C; famous carrier 'Typhoid Mary' (Mary Mallon).
- Pneumonia: Streptococcus pneumoniae and Haemophilus influenzae; infects the alveoli.
- Common cold: Rhino viruses; infect nose and respiratory passage but NOT the lungs; lasts 3-7 days.
- Malaria: Plasmodium; female Anopheles vector; malignant malaria = P. falciparum; haemozoin causes recurring chills/fever; needs two hosts.
- Amoebiasis: Entamoeba histolytica (large intestine); houseflies are mechanical carriers; Filariasis: Wuchereria bancrofti and W. malayi.
- Ringworm fungi: Microsporum, Trichophyton, Epidermophyton.
- Dengue and chikungunya are spread by Aedes mosquitoes; Gambusia fish are biological control for mosquito larvae.
- Smallpox completely eradicated by vaccination (WHO declared global eradication in 1980); India certified polio-free in 2014.
- Innate immunity = four barriers (physical, physiological, cellular, cytokine); interferons are the cytokine barrier.
- Antibody = H2L2 (2 heavy + 2 light chains); classes IgA, IgM, IgE, IgG; B-cells = humoral, T-cells = cell-mediated immunity and graft rejection.
Memory tricks remember it for good
Traps to avoid
- Common cold (Rhino virus) infects the nose and upper respiratory passage but NOT the lungs - don't confuse it with pneumonia, which floods the alveoli.
- Only the FEMALE Anopheles transmits malaria, while Aedes (not Anopheles or Culex) spreads dengue and chikungunya - match vector to disease carefully.
- Innate immunity is non-specific and present at birth; acquired immunity is specific and has memory - don't swap these traits.
- B-lymphocytes give antibodies/humoral immunity; T-lymphocytes give cell-mediated immunity and graft rejection - mixing up the cells is a classic error.
- Pneumonia here is bacterial (Streptococcus pneumoniae/Haemophilus influenzae) while common cold is viral - don't assume both are viral.
- 'Typhoid Mary' (Mary Mallon) was a carrier, not a patient or discoverer; 'Widal' is a test, not a person or disease - watch the name confusion.
Exam focus
🧠 Prelims angles
- Disease-pathogen-vector matching (typhoid/Salmonella, malaria/Plasmodium-Anopheles, filariasis/Wuchereria, ringworm/fungi).
- Diagnostic test pairing: Widal test linked to typhoid.
- Types of immunity: innate vs acquired; the four innate barriers; interferon as the cytokine barrier.
- Antibody structure (H2L2) and classes (IgA, IgM, IgE, IgG); humoral vs cell-mediated immunity.
- Vector-borne disease vectors: Anopheles (malaria), Aedes (dengue, chikungunya), and biological control by Gambusia fish.
- Vaccine-eradicable/preventable diseases: smallpox eradicated; polio, diphtheria, tetanus, pneumonia controlled.
✍️ Mains angles GS-III
- India's fight against vector-borne diseases (malaria, dengue, chikungunya, filariasis) - strategy and gaps.Link the chapter's vector-control measures (source reduction, Gambusia, nets) to integrated vector management, NVBDCP and urban sanitation.
- Vaccination and immunisation as a public-health success - from smallpox eradication to polio-free India.Use smallpox and polio examples to argue immunisation's value; connect to UIP and Mission Indradhanush.
- Biotechnology's role in newer, safer vaccines and disease control.Cite NCERT's note on biotech vaccines; discuss recombinant vaccines, affordability and self-reliance.
- Defining health beyond the absence of disease - preventive healthcare and social determinants.Use the WHO 'complete well-being' definition and link genetic, infection and lifestyle determinants to public-health policy.
Last-minute revision tick as you recall
- Health = physical + mental + social well-being (WHO), not just absence of disease.
- Typhoid - Salmonella typhi - Widal test - Typhoid Mary; common cold - Rhino virus - not lungs.
- Pneumonia - alveoli - Streptococcus/Haemophilus.
- Malaria - Plasmodium - female Anopheles - P. falciparum fatal - haemozoin - two hosts.
- Amoebiasis - Entamoeba histolytica - houseflies; Filariasis - Wuchereria; Ringworm - Microsporum/Trichophyton/Epidermophyton.
- Innate = 4 barriers (physical, physiological, cellular, cytokine/interferon); non-specific, from birth.
- Acquired = specific + memory; primary (weak) then secondary/anamnestic (strong).
- Antibody = H2L2; classes IgA, IgM, IgE, IgG; B-cell = humoral, T-cell = CMI/graft rejection.
- Vectors: Anopheles-malaria, Aedes-dengue/chikungunya; Gambusia eats larvae; smallpox eradicated, India polio-free 2014.
Distilled from NCERT Class 12 · Biology (Class 12) for UPSC. Always cross-check facts with the original NCERT.