ScienceCBSE Class 8 · Health: The Ultimate Treasure

Health, Pathogens and Types of Diseases: Class 8 Science

Updated 17 October 2026 · Fact-checked

Health is a state of complete physical, mental and social well-being, not merely the absence of disease. Diseases are of two types: communicable diseases, caused by pathogens (bacteria, viruses, fungi, protozoa, worms) and passing from person to person, and non-communicable diseases such as diabetes, cancer and asthma, linked to lifestyle, diet and environment.

Understand Health, Pathogens and Types of Diseases

What is health? The WHO (the world health body of the United Nations) defines health as a state of complete physical, mental and social well-being, and not merely the absence of disease. Physical, mental and social are the three aspects of health. A healthy person can perform tasks efficiently, cope with difficult situations and adjust well with peers and society. In Activity 3.1 a lonely student who spent too much time on a phone developed headaches, weight loss and poor sleep, so habits and surroundings affect well-being. Ayurveda, in the book's heritage note, speaks of balance of body, mind and surroundings, with daily and seasonal routines (dinacharya and ritucharya).

Staying healthy. Health depends on lifestyle and environment. Good habits: balanced diet with fruits, vegetables and whole grains; physical activity; enough sleep; limited screen time; yoga or pranayama; saying no to tobacco, alcohol and addictive drugs; personal hygiene; time with family and friends. A clean surrounding with clean air and water also matters; the Air Quality Index (AQI) tells how clean the air is.

Symptom versus sign. A symptom is what we feel (pain, tiredness, dizziness). A sign is something that can be seen or measured (fever, rash, high blood pressure, swelling).

Disease. A disease affects the normal working of the body or mind. Diseases caused by germs are due to pathogens: bacteria, viruses, fungi, worms or protozoa. Others come from poor nutrition or an unhealthy lifestyle. Some are short-lived; chronic diseases persist for more than 3 months.

Communicable diseases are caused by pathogens and spread from one person to another: typhoid, dengue, flu, chickenpox, COVID-19, common cold, measles, tuberculosis, hepatitis A, cholera, malaria and ascariasis.
Non-communicable diseases (NCDs) do not spread from person to person and are not caused by pathogens: cancer, diabetes, asthma, obesity, high blood pressure and deficiency diseases such as scurvy, anaemia and goitre. The chapter says that in India NCDs are now the most common cause of death, as people eat more processed food, exercise less and live longer.

How communicable diseases spread. Through the air (cough or sneeze), direct contact (handshakes), indirect contact (sharing towels), contaminated food and water, and through vectors, insects such as mosquitoes and houseflies, that carry pathogens. Table 3.1 lists the causal agents: common cold, influenza, chickenpox, measles and hepatitis A, dengue are caused by viruses; tuberculosis, cholera, typhoid by bacteria; malaria by protozoa; ascariasis by roundworms. Parasites live in or on another living being, and worms in the intestine feed on our nutrients.

Prevention. Wash hands with soap and water, cover the mouth and nose while coughing, wear a mask in crowds, do not share personal items, keep home, food and water clean, drink boiled water, use mosquito nets and repellents, and rest at home when unwell. The chapter's example of a community sanitation campaign in Bhadrak district, Odisha, shows that better sanitation reduced diarrhoea and infections in children.

Immunity, vaccines and antibiotics are in the next topic.

Health, Pathogens and Types of Diseases: formulas and key facts

WHO definition of health: Complete physical, mental and social well-being | Not merely the absence of disease; Three aspects: physical, mental, social.
Symptom and sign: $S y m pt o m = w ha tw e f ee l (p ain, t i r e d n ess) ∣ S i g n = see n or m e a s u r e d (f e v er, r a s h, hi g hb l oo d p r ess u r e, s w e l l in g)$; A doctor uses both.
Types of disease: $C o mm u ni c ab l e (p a t h o g e n s, s p r e a d) ∣ N o n - co mm u ni c ab l e (l i f es t y l e, d i e t, e n v i r o nm e n t, d e f i c i e n cy)$; Chronic means lasting more than 3 months.
Pathogens and examples: $V i r u s : f l u, c hi c k e n p o x, m e a s l es, h e p a t i t i s A, d e n g u e ∣ B a c t er ia : t u b er c u l os i s, c h o l er a, t y p h o i d ∣ P r o t oz o a : ma l a r ia ∣ W or m s : a sc a r ia s i s$; From Table 3.1.
Modes of spread: Air
Contact
Contaminated food and water
Vectors (mosquitoes, houseflies); Insects that carry pathogens are vectors.

How to solve Health, Pathogens and Types of Diseases questions

Use this method to classify or analyse disease questions.

1Ask: is a pathogen involved? If yes, the disease is communicable; if no, non-communicable.
2Name the pathogen group from Table 3.1 and the site of infection.
3Decide how it spreads: air, contact, food and water, or vector.
4Choose preventive steps that block that route.
5For data questions (tables or bar graphs), read the highest, lowest and total, and link seasons to the cause.
6Write short answers using the textbook words: symptom, sign, pathogen, vector.

Health, Pathogens and Types of Diseases in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Reading a disease-case table

Suppose a clinic records dengue cases for the first six months as 10, 12, 15, 18, 22 and 40 (a made-up set of numbers). The clinic wants to plan for the next months.

(a) Find the total cases in six months. (b) Find the mean per month, correct to one decimal place. (c) By what percentage did cases in the last month rise over the previous month? (d) Name one preventive step the community can take before the peak.

Try it first, then show the solutionSolution

Total = 10 + 12 + 15 + 18 + 22 + 40 = 117.
Mean = 117 ÷ 6 = 19.5.
Rise = 40 − 22 = 18; percentage = 18 ÷ 22 × 100 ≈ 81.8%.
Remove stagnant water where mosquitoes breed, use nets and repellents.

Answer: (a) 117; (b) 19.5 cases; (c) about 81.8%; (d) controlling mosquito breeding.

Memory hook: Rising bars before the rains: act before the peak.

Classify the diseases at a camp

Imagine a health camp lists: cold, diabetes, asthma, typhoid, scurvy, chickenpox, malaria.

(a) Sort them into communicable and non-communicable. (b) Which one is a deficiency disease and which nutrient is lacking? (c) Name the vector for malaria and the pathogen group.

Try it first, then show the solutionSolution

Communicable: cold, typhoid, chickenpox, malaria (pathogens). Non-communicable: diabetes, asthma, scurvy.
Scurvy is a deficiency disease; it is caused by lack of vitamin C (from earlier classes; the chapter only names scurvy as a deficiency disease).
Malaria is caused by protozoa and spread by mosquitoes.

Answer: (a) Cold, typhoid, chickenpox, malaria; diabetes, asthma, scurvy; (b) scurvy, vitamin C (earlier knowledge); (c) mosquito, protozoa.

Memory hook: Pathogen present: it spreads. No pathogen: lifestyle or lack.

Tricks to remember Health, Pathogens and Types of Diseases

Short memory aids. Say each one aloud once, then use it on a question the same day.

PMS of health
Physical, Mental, Social: the three sides of health.
Use it when: WHO definition.
Symptom = Sense, Sign = See
You sense a symptom; a doctor sees or measures a sign.
Use it when: Symptom versus sign.
Air, Water-food, Insects
Table 3.1 is grouped under these three routes.
Use it when: Mode of spread questions.
V for Vector
Vector = Vehicle carrying the germ, e.g. a mosquito.
Use it when: Define vector.

Health, Pathogens and Types of Diseases: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Define health · 2 marks · Short answerBoard level

Give the WHO definition of health in your own words and name its three aspects.

Try it first, then show the solutionSolution

Health is a state of complete physical, mental and social well-being, not merely the absence of disease. (1)
The three aspects are physical, mental and social. (1)

Answer: Complete physical, mental and social well-being, not just freedom from disease.

Example 2: Identify the NCDs · 1 mark · MCQBoard level

Which pair contains only non-communicable diseases?
(a) Typhoid and asthma
(b) Asthma and diabetes
(c) Typhoid and measles
(d) Asthma and measles

Try it first, then show the solutionSolution

Typhoid and measles are caused by pathogens.
Asthma and diabetes are not caused by pathogens and do not spread.

Answer: (b)

Example 3: Flu outbreak in school · 5 marks · Long answerHarder board level

Several students in a school have flu. (a) How does flu spread? (1) (b) Give three measures the school should take. (2) (c) A classmate coughs near you; state two ways to protect yourself and two ways to treat him kindly. (2)

Try it first, then show the solutionSolution

(a) Through air when an infected person coughs or sneezes, by direct contact, and by sharing personal items. (1)
(b) Any three: ask ill students to stay at home and rest; cover mouth and nose; wear masks in crowded places; wash hands with soap; no sharing of towels or handkerchiefs; keep classrooms clean. (2)
(c) Protect: keep a little distance, wash hands, wear a mask. Kindness: speak gently, suggest rest and a doctor, share notes later. (2)

Answer: Flu spreads through air and contact; school measures: stay home when ill, masks, hand washing; protect yourself with a mask and washing, and be considerate.

Example 4: Analysing a dengue bar graph · 4 marks · Case-basedHarder board level

Read the passage and answer.

A hospital in a made-up town records monthly dengue cases over a year:
Jan 8, Feb 9, Mar 12, Apr 16, May 25, Jun 44, Jul 70, Aug 72, Sep 60, Oct 28, Nov 24, Dec 14.
(a) Which three months had the highest cases, and what is their total? (1)
(b) Which month had the lowest? (1)
(c) Find the total for the year. (1)
(d) Give one environmental reason for a peak and one preventive step before it. (1)

Try it first, then show the solutionSolution

(a) July (70), August (72) and September (60); total = 202. (1)
(b) January, 8 cases. (1)
(c) 8 + 9 + 12 + 16 + 25 + 44 = 114; 70 + 72 + 60 = 202; 28 + 24 + 14 = 66; total = 114 + 202 + 66 = 382. (1)
(d) Standing water in the rainy months gives mosquitoes places to breed (an inference). Preventive: remove stagnant water, use nets and repellents before the peak. (1)

Answer: (a) Jul, Aug, Sep (202); (b) January (8); (c) 382; (d) stagnant water breeds mosquitoes; remove it before the peak.

Example 5: Is it communicable? · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

Anil says: 'Diabetes spreads if you sit next to a person who has it.' Beena says: 'Scurvy is caused by a germ because it makes people ill.' Chetan says: 'Malaria spreads through a mosquito.'
(a) Evaluate Anil's statement. (1)
(b) Evaluate Beena's statement. (1)
(c) Evaluate Chetan's statement and name the pathogen group. (1)
(d) Explain the word vector with one more example from the chapter. (1)

Try it first, then show the solutionSolution

(a) Wrong. Diabetes is non-communicable; it is linked to hormones, diet, lack of activity and obesity, not pathogens. (1)
(b) Wrong. Scurvy is a deficiency disease (lack of a nutrient) and is non-communicable. (1)
(c) Right, the mosquito is a vector; the pathogen is a protozoan. (1)
(d) A vector is an insect that carries pathogens; houseflies are another example. (1)

Answer: (a) Wrong; (b) wrong; (c) right, protozoa; (d) vector = insect carrying a pathogen, e.g. housefly.

Where students lose marks in Health, Pathogens and Types of Diseases

The slips examiners see most often, and what to write instead.

Mistake: Defining health as the absence of disease.
The common idea of health is 'not being ill'.
Wrong working
Health means not having any disease.
Right working
Health is complete physical, mental and social well-being, not merely absence of disease.
Do this: Include all three aspects.
Usually costs: 1 mark.
Mistake: Confusing symptom and sign.
Both indicate illness.
Wrong working
Fever is a symptom because we feel hot.
Right working
Fever is a sign (it is measured); tiredness is a symptom.
Do this: Sign can be measured or seen.
Usually costs: 1 mark.
Mistake: Calling non-communicable diseases infectious.
Students assume every disease is caused by germs.
Wrong working
Diabetes is caused by bacteria.
Right working
Diabetes is non-communicable: linked to hormones, diet and lifestyle.
Do this: Check for a pathogen.
Usually costs: 1 mark.
Mistake: Writing virus for malaria or bacteria for dengue.
Table 3.1 is not revised.
Wrong working
Malaria is a viral disease.
Right working
Malaria is caused by protozoa; dengue by a virus.
Do this: Learn the table by pathogen group.
Usually costs: 1 mark.

Exam tips

Quote the WHO definition with all three aspects.
Classify diseases by asking whether a pathogen is involved.
In prevention answers, match the step to the route of spread (boiled water for water-borne diseases, nets for mosquito-borne).
In graph questions show the numbers used and add units (cases per month).
Do not write a cure for a disease the chapter does not discuss; stick to Table 3.1.

Practice questions on Health, Pathogens and Types of Diseases

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
Differentiate between a symptom and a sign with one example each.
Hint
Feel versus measure.
Answer
A symptom is what a person feels, e.g. pain; a sign can be seen or measured, e.g. fever or rash (1 mark each).
Q2 · 1 mark · MCQBoard level
Which of the following is a vector of disease?
(a) Honey bee
(b) Mosquito
(c) Earthworm
(d) Butterfly
Hint
Think of malaria and dengue.
Answer
(b)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Boiling drinking water helps prevent cholera.
Reason (R): Cholera is caused by bacteria that can spread through contaminated water and food.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Link the preventive step to the route of spread.
Answer
(a) Cholera bacteria spread through water and food, so boiled water breaks the route.
Q4 · 4 marks · Short answerHarder board level
A clinic records the following cases in 6 months: 8, 12, 14, 20, 26, 40. (a) Find the total. (b) Find the mean. (c) Find the percentage increase from the 1st to the 6th month. (d) Name two measures to control the spread if this were a mosquito-borne disease.
Hint
Percentage increase = (change ÷ original) × 100.
Answer
(a) 8 + 12 + 14 + 20 + 26 + 40 = 120. (b) 120 ÷ 6 = 20. (c) (40 − 8) ÷ 8 × 100 = 400%. (d) Nets and repellents; stopping water from standing (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A student says: 'Since I am not sick, I am healthy.' Another has no disease but is lonely, sleeps badly and spends nine hours a day on a phone.
(a) Is the first claim complete according to WHO? (b) Name the aspects of health affected in the second student. (c) Suggest three habits that can help. (d) Is a chronic disease one that lasts a short time? Explain.
Hint
Think of physical, mental and social aspects.
Answer
(a) No: health is complete physical, mental and social well-being, not just absence of disease. (b) Mental and social (and physical through poor sleep). (c) Limit screen time, sleep enough, spend time with friends and family, exercise. (d) No: chronic diseases last more than 3 months (1 mark each).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is a pathogen?Show answer
A disease-causing organism such as a bacterium, virus, fungus, protozoan or worm.
2.What is a vector?Show answer
An insect, such as a mosquito or housefly, that carries pathogens.
3.Give three non-communicable diseases.Show answer
Cancer, diabetes, asthma.
4.Which disease is caused by a protozoan?Show answer
Malaria.
5.What is a chronic disease?Show answer
One that persists for more than 3 months.
6.What does the AQI show?Show answer
How clean the air is.

Health, Pathogens and Types of Diseases: frequently asked questions

Are deficiency diseases communicable?

No. Diseases like scurvy, anaemia and goitre are caused by a lack of specific nutrients and are non-communicable.

Why have NCDs become common in India?

According to the chapter, because of changes in how people live: more processed food, less exercise and longer lives.

What is the difference between a parasite and a pathogen?

A pathogen is any disease-causing organism; a parasite is an organism that lives in or on another living being, such as intestinal worms.

How does hand washing prevent disease?

Washing hands with soap and water removes pathogens before they can enter the body or be passed to others.