- Health is not merely the absence of disease — it is a state of complete physical, mental and social well-being (WHO definition).
- Disease means the body or its parts are not functioning properly; there are signs (measurable, e.g. fever confirmed by thermometer) and symptoms (felt by patient, e.g. headache, nausea).
- Acute diseases last a short time (common cold, typhoid if caught early); chronic diseases persist for months or years (tuberculosis, diabetes, elephantiasis).
- Infectious (communicable) diseases are caused by microorganisms called pathogens; non-infectious diseases are caused by other factors (genetic, nutritional, lifestyle).
- Prevention is better than cure: general prevention (hygiene, sanitation, nutrition) and specific prevention (vaccination / immunisation).
- Board weightage: ~4 marks/year — typically a 2-mark question on pathogens or modes of transmission and a 2-mark question on prevention or immunisation.
1. What is Health?
Health is not simply the absence of disease. The World Health Organisation (WHO) defines health as "a state of complete physical, mental and social well-being." Three dimensions must all be satisfied simultaneously:
- Physical health: all body organs and systems function properly — no illness, adequate energy levels, good physical fitness.
- Mental health: emotional stability, ability to think clearly, handle stress, and maintain positive personal relationships.
- Social health: ability to function productively within society, access to basic resources such as food and shelter, absence of discrimination and oppression.
This definition means our health is shaped not just by our biology but also by our social and economic environment. Poverty, inequality, poor housing and lack of sanitation all damage health even without any germ being present. A person living in a slum with dirty water is in poor health even if no specific disease is currently diagnosed.
Well-being vs disease-free: a person might be disease-free yet not healthy. For example, a person suffering from severe depression or chronic stress is not healthy even if no infection is present. Health is a positive, wide-ranging concept, not just a biological state.
NCERT asks students to think: does a road-worker living near a noisy, polluted highway have the same health as someone living in a quiet, clean neighbourhood? The answer is no. Environmental stressors — noise, air pollution, poor housing, insecure employment — are all health factors. This is why community-level actions (cleaning drains, planting trees, regulating pollution) are genuine health measures.
2. Personal and Community Health
Individual health is deeply connected to the health of the surrounding community. The availability and quality of basic public services largely determine whether individuals can remain healthy.
- Clean water supply: dirty drinking water causes waterborne diseases such as cholera, typhoid and hepatitis A, which spread rapidly through a population.
- Garbage and waste disposal: open garbage dumps breed mosquitoes (malaria, dengue, chikungunya) and rats (leptospirosis, plague).
- Ventilation and housing: overcrowded, poorly ventilated rooms accelerate the airborne spread of tuberculosis, influenza and COVID-19.
- Sewage treatment: untreated sewage contaminates water sources — the faeco-oral route spreads some of the world's most dangerous diseases.
- Public health services: government immunisation drives, food safety laws and vector-control programmes (spraying larvicides in drains) are examples of community health measures that benefit everyone.
NCERT emphasises that public health infrastructure — hospitals, community health workers, clean water systems, functional sanitation — is not a luxury but a necessity for preventing disease at the population level.
Q: Are the problems of the mosquito menace and garbage disposal related to disease and health? Ans: Yes — garbage provides breeding grounds for mosquitoes, which are vectors (carriers) of malaria, dengue and chikungunya. Controlling garbage disposal directly reduces mosquito populations and therefore reduces the incidence of these vector-borne diseases. This shows that environment management is a critical component of disease prevention.
3. Disease and Its Causes
A disease is any condition in which the normal structure or functioning of the body is disturbed. NCERT distinguishes between the immediate cause (the microbe or agent that enters the body) and contributory causes (the underlying social, nutritional and environmental conditions that made the person vulnerable).
Non-infectious (non-communicable) diseases — not caused by pathogens and cannot spread from person to person:
- Nutritional deficiency diseases: scurvy (vitamin C deficiency), rickets (vitamin D), anaemia (iron or B12), kwashiorkor (severe protein deficiency), marasmus (overall calorie deficiency).
- Genetic/hereditary diseases: haemophilia, colour blindness, sickle-cell anaemia, Down syndrome — caused by defective genes inherited from parents.
- Degenerative diseases: arthritis, coronary artery disease, cataract — result from progressive wear and age-related deterioration of tissues.
- Lifestyle diseases: type 2 diabetes, hypertension, obesity, some cancers — linked to diet, physical inactivity and habits such as tobacco use.
- Cancer: uncontrolled cell division; can be triggered by carcinogens such as tobacco smoke, ultraviolet radiation or certain chemicals.
These diseases require a different prevention approach compared to infectious diseases: dietary changes, regular exercise, genetic counselling and lifestyle modification — not antibiotics or vaccines.
4. Acute vs Chronic Diseases
A key NCERT distinction tested regularly in board exams:
| Feature | Acute Disease | Chronic Disease |
| Duration | Short (days to a few weeks) | Long (months to many years, sometimes lifelong) |
| Effect on body weight and strength | Little lasting effect; full recovery is usual | Progressive drastic loss of weight, muscle mass and overall strength |
| Examples | Common cold, acute gastroenteritis, mild typhoid | Tuberculosis, diabetes, elephantiasis (filariasis), arthritis, kala-azar |
| Economic and social impact | Few days of work or school missed | Long-term disability, sustained loss of income, high treatment costs |
NCERT emphasises that chronic diseases are far more damaging to a person's overall health because they continuously drain the body over an extended period, affecting the ability to work, study and maintain normal life. Treatment of chronic disease is also expensive and prolonged.
Q: Why are we generally more worried about the effects of chronic diseases than acute diseases? Ans: Chronic diseases last for months or years, damage vital organs progressively, and lead to long-term debilitation — including severe loss of body weight, constant fatigue, weakening of muscles and loss of the ability to earn a livelihood. An acute disease, by contrast, is usually short-lived and the person recovers fully. The sustained drain of a chronic disease on health, finances and quality of life makes it far more worrying.
5. Infectious Diseases — Pathogens
Infectious diseases are caused by pathogens — disease-causing microorganisms that invade a host body and disturb normal function. NCERT classifies pathogens into five main groups:
| Pathogen type | Nature | Diseases caused (NCERT examples) |
| Bacteria | Prokaryotic unicellular organisms | Typhoid (Salmonella typhi), tuberculosis (Mycobacterium tuberculosis), cholera (Vibrio cholerae), diphtheria, anthrax, acne (Propionibacterium acnes) |
| Viruses | Non-cellular; can only replicate inside a living host cell using the host's own machinery | Common cold (rhinovirus), influenza, dengue, AIDS (HIV), polio, rabies, hepatitis A/B, measles, chickenpox |
| Fungi | Eukaryotic; many are multicellular; thrive in warm moist conditions | Ringworm / tinea (Dermatophytes), athlete's foot, oral thrush (Candida albicans), skin and nail infections |
| Protozoa | Unicellular eukaryotes with complex life cycles | Malaria (Plasmodium), kala-azar / leishmaniasis (Leishmania donovani), sleeping sickness (Trypanosoma), amoebic dysentery (Entamoeba histolytica) |
| Helminths (worms) | Multicellular eukaryotes — flatworms and roundworms; visible to naked eye at adult stage | Elephantiasis / filariasis (Wuchereria bancrofti — transmitted by Culex mosquito), intestinal roundworm (Ascaris lumbricoides), tapeworm (Taenia solium) |
Key NCERT point about viruses: Unlike bacteria, viruses are not cells and have no independent metabolism. They replicate only inside a living host cell by hijacking its ribosomes and enzymes. This is why antiviral drugs are much harder to develop than antibiotics — any drug that blocks a viral process risks harming the host cell as well. Most currently available antivirals do not cure viral infections outright but only slow replication.
Antibiotics such as penicillin work by blocking bacterial cell-wall synthesis — a process that human cells and viruses do not carry out. The common cold is caused by a rhinovirus that has no cell wall and uses the host cell's own machinery to reproduce. Antibiotics therefore have absolutely nothing to block in a viral infection. Taking antibiotics for a cold not only fails to help but also kills beneficial gut bacteria and accelerates the development of antibiotic-resistant strains of bacteria — a major global health problem.
6. Modes of Transmission
Pathogens travel from one host to another through several well-defined routes. NCERT lists the following modes:
- Air (droplet / airborne transmission): when an infected person sneezes, coughs or talks, they release tiny droplets and aerosol particles loaded with pathogens. These can travel metres through the air and enter the respiratory tract of nearby people. Diseases spread this way: common cold, influenza, tuberculosis (TB), measles, whooping cough, COVID-19. Prevention: covering the mouth and nose when coughing, good room ventilation, wearing masks.
- Water and food (faeco-oral route): pathogens in the faeces of a sick person contaminate drinking water or food (e.g. via unwashed hands or flies). Diseases: cholera, typhoid, amoebic dysentery, hepatitis A, hepatitis E, polio. Prevention: boil or treat drinking water, maintain proper sewage systems, wash hands before preparing or eating food.
- Sexual contact: pathogens present in semen, vaginal fluids or blood are transmitted during unprotected sexual intercourse. Diseases: AIDS (HIV), syphilis (Treponema pallidum), gonorrhoea (Neisseria gonorrhoeae), genital herpes. Prevention: condom use, mutual monogamy, regular testing.
- Blood and body fluids (parenteral route): sharing needles during intravenous drug use, infected blood transfusions, contaminated surgical instruments, or mother-to-child transmission during childbirth or breastfeeding. Diseases: HIV/AIDS, hepatitis B and C. Prevention: screening all donated blood, using sterile single-use needles, safe delivery practices.
- Vectors (carrier organisms): certain animals carry pathogens and transmit them through biting without becoming visibly ill themselves. Key vectors:
- Female Anopheles mosquito — transmits malaria (Plasmodium falciparum and others).
- Aedes aegypti mosquito — transmits dengue fever, chikungunya, yellow fever and Zika.
- Culex mosquito — transmits filariasis / elephantiasis (Wuchereria bancrofti).
- Dogs (and other mammals) — transmit rabies through bites (the virus travels via nerves to the brain).
- Flies — mechanically contaminate food with typhoid and dysentery pathogens (not true vector transmission but still important).
- Direct skin or surface contact: fungal infections such as ringworm (tinea) spread by touching infected skin or contaminated surfaces, towels and clothing. Prevention: do not share personal items; keep skin clean and dry.
Q: A patient has a running nose and cough with the common cold but not with typhoid. Why do symptoms differ? Ans: In the common cold, the rhinovirus infects the lining of the nose and upper respiratory tract. The body's immune response in those specific tissues — increased mucus production (runny nose), inflammation and irritation (cough) — produces respiratory symptoms. In typhoid, Salmonella typhi infects the lining of the small intestine and the associated lymph nodes. The immune response in those tissues produces fever, abdominal pain and intestinal ulceration — not respiratory symptoms. The symptoms of a disease always reflect which organ or tissue is infected and how the immune system responds there.
7. Organ-Specific and Tissue-Specific Effects
Different pathogens target different organs or tissue systems depending on how they enter the body and which cells they can infect. The tissue that is affected determines the symptoms the patient shows. NCERT key examples:
- Lungs: Mycobacterium tuberculosis infects lung tissue, causing persistent cough, blood in sputum, progressive breathlessness and weight loss (tuberculosis / TB).
- Small intestine: Salmonella typhi infects intestinal mucosa causing high fever, abdominal pain, rose-coloured spots and intestinal ulcers (typhoid). Vibrio cholerae releases a toxin that causes profuse watery diarrhoea (cholera).
- Liver: Hepatitis viruses (A, B, C, E) infect and destroy liver cells, causing jaundice (yellow skin and eyes due to build-up of bilirubin), fatigue, nausea and liver enlargement.
- Nervous system / brain: Rabies virus travels along nerve cells from the site of the bite to the brain, causing hydrophobia, aggression and convulsions — almost always fatal once symptoms appear. Japanese encephalitis virus causes brain inflammation. HIV can cross the blood-brain barrier, causing dementia in late-stage AIDS.
- Red blood cells and liver: Plasmodium (malaria parasite) first multiplies in liver cells, then enters and ruptures red blood cells. Each cycle of RBC rupture releases toxins, causing the characteristic waves of high fever, chills and sweating. The destruction of RBCs also causes anaemia.
- Immune cells (lymphocytes): HIV specifically targets CD4+ T-helper lymphocytes — the very cells that coordinate the immune response. Destroying these cells progressively collapses the immune system, leading to AIDS (Acquired Immunodeficiency Syndrome) where the body can no longer fight even mild infections (opportunistic infections such as Pneumocystis pneumonia and candidiasis become life-threatening).
- Skin and soft tissue: Dermatophyte fungi infect the outer keratinised skin layer causing itchy circular patches (ringworm / tinea corporis). Leishmania protozoa infect skin and internal organs (liver, spleen) in kala-azar, causing dark discolouration of skin and severe wasting.
- Lymphatic system: Wuchereria bancrofti (filarial worm) lives in the lymphatic vessels, blocking lymph drainage and causing massive swelling of limbs and genitals (elephantiasis).
NCERT principle — immune response is part of the symptom: many symptoms such as fever, pain and swelling are not caused directly by the pathogen destroying tissue but by the body's own immune response (inflammation, increased temperature, mucus secretion) trying to fight the infection. Fever, for example, is the immune system raising body temperature to inhibit microbial replication and speed up immune-cell activity.
8. Principles of Treatment
NCERT outlines two main approaches to treating an infectious disease once it has occurred:
- Reduce the effect / treat symptoms: medicines such as antipyretics (reduce fever — paracetamol), analgesics (reduce pain) and anti-inflammatory drugs make the patient more comfortable and prevent dangerous complications like very high fever causing brain damage. Adequate rest allows the immune system to divert all resources to fighting the pathogen. Oral rehydration solution (ORS) is critical in diarrhoeal diseases to prevent fatal dehydration.
- Kill the cause / target the pathogen:
- Antibiotics — for bacterial infections. They target biochemical processes that bacteria possess but human cells do not: beta-lactams (penicillin, amoxicillin) block bacterial cell-wall synthesis; aminoglycosides (streptomycin) and macrolides block bacterial ribosomes; fluoroquinolones (ciprofloxacin) block bacterial DNA replication. Key rule: antibiotics have zero effect on viruses.
- Antivirals — for viral infections. Much harder to develop. Examples: acyclovir (herpes), oseltamivir/Tamiflu (influenza), highly active antiretroviral therapy (HAART) for HIV. They slow viral replication rather than destroying the virus outright.
- Antifungals — for fungal infections (e.g. clotrimazole cream for ringworm, fluconazole for systemic fungal infections).
- Antiprotozoals — for protozoal diseases (e.g. chloroquine / artemisinin for malaria, metronidazole for amoebic dysentery, sodium stibogluconate for kala-azar).
- Anthelmintics — for worm infestations (e.g. albendazole, mebendazole for intestinal worms; diethylcarbamazine/DEC for filariasis).
NCERT key message: treatment is limited in scope and usefulness because (a) by the time a person is treated, their body has already suffered damage, and (b) the person may have already transmitted the disease to others. This is the strongest argument for prevention being always better than cure.
Q: Why are antibiotics not effective for treating viral diseases? Ans: Antibiotics specifically block bacterial biochemical processes such as cell-wall synthesis and bacterial protein synthesis — processes that viruses do not carry out. Viruses are not cells and have no independent metabolism; they use the host cell's own ribosomes and enzymes to replicate. Because viruses lack the structures that antibiotics target, antibiotics find nothing to act on and are completely ineffective against viral infections like the common cold, dengue or AIDS.
9. Principles of Prevention — General Measures
NCERT distinguishes between general prevention (reducing exposure to any pathogen) and specific prevention (targeting a specific pathogen via vaccines, covered in section 10).
General prevention measures:
- Personal hygiene: regular and thorough hand washing with soap (especially before eating, after using the toilet and after contact with sick people), keeping the body clean, trimming fingernails (pathogens hide under nails), covering the mouth and nose when coughing or sneezing. These break the transmission chain at the personal level.
- Safe drinking water: boiling water for at least one minute kills waterborne pathogens. Chlorination and filtration at the community level prevent cholera, typhoid, hepatitis A and polio.
- Proper food hygiene: cooking food thoroughly (heat kills most pathogens), avoiding stale or uncovered food, washing fruits and vegetables before eating, avoiding food from visibly unhygienic sources.
- Good sanitation: constructing and using toilets (no open defecation), maintaining closed drainage systems and proper sewage treatment before discharge. Open defecation is one of the most important risk factors for diarrhoeal disease mortality in children in India and worldwide.
- Vector control: eliminating breeding sites for mosquitoes — emptying or covering containers that hold stagnant water (flower pots, tyres, coolers, roof tanks), clearing blocked drains, using mosquito nets (especially for young children and during sleep), applying DEET-based repellents on exposed skin. These measures prevent malaria, dengue and filariasis.
- Isolation of infected persons: keeping patients with airborne diseases (influenza, TB, chickenpox) away from healthy individuals prevents respiratory droplet transmission. Patients with active TB must wear masks and complete their full course of antibiotics (under DOTS — Directly Observed Treatment Short-course in India).
- Strengthening the immune system through good nutrition: a well-nourished immune system is far more effective at fighting pathogens than a malnourished one. Vitamins A and C and the mineral zinc are particularly important for immune function. Adequate protein intake is also essential as antibodies are proteins. Sufficient sleep and physical activity further support immune health.
NCERT emphasis: clean water supply and proper sewage disposal are historically the most powerful public health interventions — the dramatic fall in death rates from typhoid and cholera in industrialised countries in the nineteenth and early twentieth centuries occurred largely because of clean-water infrastructure, before antibiotics were even discovered.
10. Immunisation — Vaccines and Immune Memory
Specific prevention means using vaccination to make the immune system capable of destroying a particular pathogen before it can cause disease. This is the most powerful tool in infectious-disease prevention.
How the immune system normally works:
- When a pathogen enters the body, specialised white blood cells (lymphocytes) recognise its surface molecules called antigens.
- B-lymphocytes produce antibodies — proteins precisely shaped to bind to and neutralise the specific antigen, marking the pathogen for destruction by other immune cells.
- T-lymphocytes directly attack and kill infected cells and co-ordinate the overall immune response.
- After the infection is cleared, some of these immune cells remain in the body as memory cells. On re-exposure to the same pathogen, these memory cells respond within hours — far faster and in far greater numbers than the first response — so the pathogen is destroyed before disease develops. This is called acquired / adaptive immunity.
Vaccination / Immunisation:
- A vaccine introduces a weakened (attenuated), killed, or partial (subunit/mRNA) form of a pathogen into the body. This does not cause the full disease but is enough to trigger an immune response and create long-lasting memory cells.
- When the real, live pathogen later enters a vaccinated person, the pre-existing memory cells mount an immediate, massive response — neutralising the pathogen before it can replicate to disease-causing levels.
- Important vaccines and the diseases they prevent (NCERT / UIP India):
- BCG vaccine — tuberculosis (given at birth in India)
- OPV / IPV — polio (oral polio drops or injected)
- DPT — diphtheria, pertussis (whooping cough) and tetanus
- MMR — measles, mumps and rubella
- Hepatitis B vaccine — hepatitis B
- Typhoid vaccine — typhoid fever
- Rabies vaccine — given after a dog or animal bite
- Influenza vaccine — seasonal flu
- Herd immunity: if a large enough proportion of the population (usually 70–95% depending on the disease) is vaccinated, the pathogen cannot find enough susceptible hosts to spread, and even unvaccinated individuals (infants too young to be vaccinated, immunocompromised patients) are indirectly protected. This collective protection is called herd immunity.
- Disease eradication through vaccination: if a disease has no non-human animal reservoir and an effective vaccine exists, universal vaccination can eliminate the pathogen from the world entirely. Smallpox was declared globally eradicated by WHO in 1980 — the first (and so far only) human infectious disease to be eradicated. Polio is close to eradication. Measles could also be eradicated if vaccination coverage remains high.
- India's Universal Immunisation Programme (UIP): the Government of India provides free vaccines through its UIP for BCG, OPV, DPT, MMR, Hepatitis B, Rotavirus vaccine and several others. This programme has dramatically reduced under-five child mortality in India over the past four decades.
A child who recovers from chickenpox (varicella-zoster virus infection) has, during recovery, produced specific antibodies and memory lymphocytes targeted at the varicella virus. If the same virus tries to infect this person again — perhaps years later — the memory cells immediately produce a massive antibody response within hours, destroying the virus before it can multiply enough to cause symptoms. The person is immune for life. Vaccination works on exactly this principle: by safely introducing a harmless form of the pathogen, it generates immune memory without the person having to suffer through the actual disease.
Q: What are the means by which infectious diseases are spread? Ans: Infectious diseases spread through: (1) air — via droplets from coughing and sneezing (common cold, TB, flu); (2) contaminated water or food — faeco-oral route (cholera, typhoid, hepatitis A); (3) sexual contact (HIV, syphilis, gonorrhoea); (4) blood and body fluids — transfusions, shared needles, mother-to-child (HIV, hepatitis B and C); (5) vectors — mosquito bites for malaria, dengue, elephantiasis; dog bites for rabies; (6) direct skin contact (ringworm, other fungal infections).
11. Quick Revision Summary
- Health = Physical + Mental + Social well-being — all three required; not just absence of disease (WHO).
- Community health (clean water, sanitation, vector control) directly determines individual health.
- Diseases: acute (short, mild long-term effect) vs chronic (long, drastic effect on body); infectious (pathogen, spreads) vs non-infectious (no pathogen, does not spread).
- Pathogens: Bacteria (typhoid, TB, cholera) | Viruses (cold, dengue, AIDS, polio) | Fungi (ringworm) | Protozoa (malaria, kala-azar) | Helminths (elephantiasis, tapeworm).
- Transmission: air, water/food, sexual contact, blood, vectors (mosquito/dog), direct contact.
- Antibiotics kill bacteria ONLY — completely ineffective against viruses.
- Symptoms reflect which organ/tissue is infected; fever and inflammation are partly the immune response itself.
- Treatment: symptom relief (antipyretics, rest) + kill the cause (antibiotics/antivirals/antiprotozoals).
- General prevention: hygiene, safe water, sanitation, vector control, good nutrition.
- Specific prevention: vaccination — introduces harmless pathogen form, creates immune memory cells, protects on future exposure.
- Smallpox eradicated in 1980 by global vaccination — proof that vaccines can completely eliminate diseases.
- The absence of any infection or disease in the body
- A state of complete physical, mental and social well-being
- Having a fit body, high energy and freedom from mental stress
- Being free from all viruses, bacteria and parasites
- Acute infectious disease caused by a virus
- Non-infectious chronic disease linked to lifestyle
- Chronic infectious disease caused by a bacterium
- Vector-borne acute protozoal disease
- Bacterium; Aedes
- Protozoan (Plasmodium); female Anopheles
- Virus; Culex
- Helminth (Wuchereria); female Anopheles
- Viruses only — because they target viral DNA
- Both bacteria and viruses — because they target nucleic acids
- Bacteria only — because they target structures absent in viruses and human cells
- Fungi only — because they target ergosterol in fungal cell membranes
- A bacterium — spread through air
- A protozoan — spread through contaminated water
- A fungus — spread by direct skin contact
- A helminth (Wuchereria bancrofti) — transmitted by Culex mosquito
- Ringworm
- Dengue fever
- Cholera
- Rabies
- The vaccine directly kills all copies of the pathogen in the body permanently
- The vaccine introduces a weakened/killed form of the pathogen, triggering immune memory cells that respond rapidly on future exposure
- The vaccine introduces antibiotics that remain stored in the body for future use
- The vaccine increases the number of red blood cells to help carry more oxygen during infection
- Malaria
- Polio
- Smallpox
- Tuberculosis
- Sharing food, utensils or drinking from the same glass
- Being bitten by infected Aedes mosquitoes
- Unprotected sexual contact and transfusion of infected blood
- Touching or hugging an infected person
- Helminth (worm) that forms circular lesions under the skin
- Bacterium that causes circular skin ulcers
- Fungus (Dermatophytes/Tinea) that infects the outer skin layer
- Protozoan that infects red blood cells in a ring-shaped pattern
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