1. Introduction — What is disease?

A disease is a condition in which the normal structure or functioning of any part of the body is disturbed, producing signs and/or symptoms. Diseases reduce the ability of an organism to function normally. For humans, disease affects physical, mental and social well-being.

2. Types of diseases

Diseases may be classified in different ways; a simple NCERT-friendly classification is:

• Infectious diseases: Caused by pathogens (disease-causing organisms) such as bacteria, viruses, fungi, protozoa and helminths (worms). They can spread from an infected person (or source) to a healthy person.
• Non-infectious diseases: Not spread from person to person — include genetic disorders, nutritional deficiencies, lifestyle diseases (diabetes, heart disease), and environmental illnesses.

3. Common pathogens — quick overview

Understanding the main agents that cause infectious diseases is central to this chapter:

• Bacteria — single-celled prokaryotes. Examples: Vibrio cholerae (cholera), Mycobacterium tuberculosis (tuberculosis). Bacterial infections can often be treated with antibiotics.
• Viruses — acellular infectious particles (DNA or RNA with protein coat). Examples: influenza virus (flu), poliovirus (polio), common cold viruses, dengue virus. Viruses need host cells to reproduce; antibiotics do not work on viruses.
• Fungi — eukaryotic organisms; cause infections like ringworm, athlete’s foot, candidiasis.
• Protozoa — single-celled eukaryotes; example: Plasmodium species (cause malaria).
• Helminths (worms) — parasitic worms causing intestinal infections (tapeworms, roundworms).

4. How infectious diseases spread — Modes of transmission

NCERT emphasises several common routes by which pathogens spread:

• Direct contact — physical contact with an infected person (touching, kissing) can spread infections such as common cold, flu.
• Contaminated food and water — ingestion of pathogens in food/water causes diseases like cholera, typhoid, hepatitis A.
• Airborne transmission — pathogens expelled in droplets when an infected person coughs/sneezes (influenza, tuberculosis).
• Vectors — animals or insects (mosquitoes, flies) transfer pathogens between hosts. Example: malaria (mosquito vector), dengue (Aedes mosquito).
• Indirect contact — touching contaminated objects (fomites) such as doorknobs, utensils or medical instruments.

5. Symptoms, signs and diagnosis

Symptoms are subjective experiences reported by the patient (e.g., headache, fatigue). Signs are objective findings noted by others or measurable (fever, rash, elevated blood pressure). Diagnosis combines patient history, signs & symptoms, laboratory tests (blood tests, cultures, imaging) and sometimes epidemiological information.

6. Body’s defence mechanisms (Immunity) — NCERT essentials

The human body has multiple layers of defence against pathogens:

1. Barriers (First line): Skin and mucous membranes act as physical barriers. Secretions like saliva, tears and mucus trap and wash away microbes. Stomach acid destroys many ingested microbes.
2. Non-specific (Innate) immunity: When pathogens enter, innate immune cells (phagocytes like macrophages and neutrophils) ingest and destroy them. Inflammation, fever and complement proteins are part of this response.
3. Specific (Adaptive) immunity: When innate mechanisms are insufficient, the adaptive system responds: B-lymphocytes produce antibodies (humoral immunity) that neutralise pathogens; T-lymphocytes (cell-mediated immunity) kill infected cells or help other immune cells. Adaptive immunity has memory — faster and stronger upon re-exposure.

7. Vaccination and immunisation

Vaccination trains the immune system by exposing it to safe forms of pathogens (killed, attenuated, or parts of the pathogen) so that memory cells form without causing disease. Examples: OPV/IPV for poliomyelitis, DPT (diphtheria, pertussis, tetanus), measles vaccine.

Key points for exams: Vaccines provide active immunity and help prevent outbreaks by creating herd immunity when a large proportion of the population is immunised.

8. Antibiotics and their correct use

Antibiotics are drugs that kill or inhibit bacteria (e.g., penicillin, tetracycline). Important NCERT facts:

• Antibiotics work only on bacteria — they do not affect viruses.
• Incorrect use (incomplete courses, overuse) leads to antibiotic resistance — bacteria evolve mechanisms to survive drugs, making infections harder to treat.
• Always use antibiotics under medical supervision and complete the full course prescribed.

9. Preventive measures and hygiene (Public health focus)

Prevention is central in public health and exam answers should emphasise practical measures:

• Personal hygiene: Regular handwashing with soap, safe food practices, boiling water where necessary, covering mouth while coughing/sneezing.
• Sanitation: Proper sewage disposal, clean drinking water supply, and safe waste management reduce waterborne diseases.
• Vector control: Eliminate stagnant water to control mosquitoes, use mosquito nets, repellents and community hygiene campaigns.
• Vaccination drives: Regular immunisation schedules protect children and adults against vaccine-preventable diseases.

10. Examples of important diseases covered in NCERT

• Malaria: Caused by Plasmodium (protozoan) transmitted by Anopheles mosquitoes; symptoms include fever, chills and anaemia; prevent by mosquito control and nets.
• Tuberculosis (TB): Caused by Mycobacterium tuberculosis; affects lungs (cough, blood in sputum, weight loss); treated with multi-drug antibiotic therapy under DOTS programme.
• Common cold and influenza: Viral infections transmitted by droplets; manage with rest, fluids and symptomatic treatment; annual flu vaccines reduce risk.
• Cholera: Bacterial disease from contaminated water causing severe diarrhoea and dehydration; prevented by clean water and sanitation; treated with rehydration and antibiotics in severe cases.

11. Role of community and government

Control of many diseases requires coordinated community and government action — vaccination programmes, surveillance of outbreaks, improving water supply and sanitation, public education, and access to healthcare. School-level awareness (handwashing, cough etiquette) prevents spread among children.

12. Key exam pointers — how to answer long/short questions

When answering, follow this approach:

1. Define the term succinctly (1–2 lines).
2. Explain causes or mechanisms in bullet points or short paragraphs.
3. Use examples (disease names, pathogens) to support your answer.
4. For prevention questions, list practical steps under headings (personal, community, medical).
5. Finish with a one-line conclusion/importance.

13. Common misconceptions to avoid

• Do not state that antibiotics cure viral infections — this is incorrect and a common mark-losing mistake.
• Vaccine-induced immunity is active immunity — differentiate from passive immunity (e.g., maternal antibodies in newborns).
• Distinguish between symptoms and causes — mentioning cause (pathogen) gains more marks than listing only symptoms.

14. Practice questions (brief)

1. Define infectious disease and give two differences between infectious and non-infectious diseases.
2. Explain two modes of transmission of infectious agents with one example each.
3. What is vaccination? How does it protect a population (two points)?
4. Why should antibiotics be used carefully? Explain antibiotic resistance in two lines.

15. Consolidated summary (one-paragraph revision)

Diseases arise when normal body functions are disturbed by pathogens (infectious) or other causes (non-infectious). Infectious diseases spread via direct contact, contaminated food/water, airborne droplets or vectors. The body defends itself with barriers, innate immunity and adaptive immunity; vaccination primes adaptive immunity to prevent many diseases. Antibiotics treat bacterial infections but not viral ones — misuse causes resistance. Prevention through hygiene, sanitation, vector control and vaccination underpins public health efforts and is the most effective strategy to reduce disease burden.

Exam-ready closing line: Understanding causes, modes of spread and prevention of diseases helps us protect individual and public health; write answers clearly, include examples, and emphasise prevention.