NCERT Textbook: Health: The Ultimate Treasure | Science Curiosity Class 8 - New NCERT PDF Download

 Page 1


28
Curiosity — Textbook of Science for Grade 8
Immersion into social media is 
spurring rise in loneliness
11.4% of India’s population is 
diabetic 
Fast food is one of the leading causes of 
diabetics 
Air pollution is one of the leading 
risk factors for health
Excessive screen watching leads to dry eye 
disease in children  
More than 100 million people 
in India are diabetic: ICMR report
Yoga reduces anxiety and 
increase concentration
Air pollution is a public 
health emergency
Increase in screen time in children leads to 
a rise in obesity, sleep disorders, and anxiety 
28.6% Indians are obese
Arrival of monsoon leads to increase in 
malaria and engue cases
Diet mantra: Reduce the amount 
of oil and sugar in diet
Regular handwashing reduces 
infection by 50%
Diabetes & heart attack are more 
prevalent in people of South-east 
Asia: WHO 
Safe drinking water reduces cases of water 
and vector-borne diseases 
FIT INDIA MOVEMENT
Reduce oil in diet by 10%
Yoga for the mind, body and soul
 
28 Curiosity — Science Textbook for Grade 8
3
Health: 
The Ultimate Treasure
Probe and ponder
 z How does your body respond to an infection such as common cold? 
 z We rarely see cases of smallpox or polio these days, but diseases 
like diabetes and heart problems are more common. Why?
 z Could climate change lead to new types of diseases?
 z How do emotions like stress or worry affect us and make us sick?   
 z Why do some groups of people get affected more than others 
during disease outbreaks?
 z Share your questions
 
 ? 
Chapter 3.indd   28 Chapter 3.indd   28 6/28/2025   5:56:27 PM 6/28/2025   5:56:27 PM
Page 2


28
Curiosity — Textbook of Science for Grade 8
Immersion into social media is 
spurring rise in loneliness
11.4% of India’s population is 
diabetic 
Fast food is one of the leading causes of 
diabetics 
Air pollution is one of the leading 
risk factors for health
Excessive screen watching leads to dry eye 
disease in children  
More than 100 million people 
in India are diabetic: ICMR report
Yoga reduces anxiety and 
increase concentration
Air pollution is a public 
health emergency
Increase in screen time in children leads to 
a rise in obesity, sleep disorders, and anxiety 
28.6% Indians are obese
Arrival of monsoon leads to increase in 
malaria and engue cases
Diet mantra: Reduce the amount 
of oil and sugar in diet
Regular handwashing reduces 
infection by 50%
Diabetes & heart attack are more 
prevalent in people of South-east 
Asia: WHO 
Safe drinking water reduces cases of water 
and vector-borne diseases 
FIT INDIA MOVEMENT
Reduce oil in diet by 10%
Yoga for the mind, body and soul
 
28 Curiosity — Science Textbook for Grade 8
3
Health: 
The Ultimate Treasure
Probe and ponder
 z How does your body respond to an infection such as common cold? 
 z We rarely see cases of smallpox or polio these days, but diseases 
like diabetes and heart problems are more common. Why?
 z Could climate change lead to new types of diseases?
 z How do emotions like stress or worry affect us and make us sick?   
 z Why do some groups of people get affected more than others 
during disease outbreaks?
 z Share your questions
 
 ? 
Chapter 3.indd   28 Chapter 3.indd   28 6/28/2025   5:56:27 PM 6/28/2025   5:56:27 PM
Chapter 3?—?Health: The Ultimate Treasure 29
3.1 Health: Is It More Than Not Falling Sick?
What do the news clippings on the notice board tell you about 
people’s health in our country? Is being healthy just about not 
having diseases? Health also includes feeling good physically, 
staying positive, and having strong relationships. A healthy person 
takes care of their body, maintains a positive mindset, and enjoys 
social life. Let us now explore what it truly means to be healthy.
Activity 3.1:  Let us read
A Grade 8 student moved to a new school in another city. With no 
friends in his new environment and busy parents, he felt lonely. To 
cope, he spent more time on his phone and social media, but this made 
him feel worse. He stopped trying to make friends, had headaches, 
lost weight, and could not sleep well. A doctor advised less screen 
time and meeting a counsellor. The school counsellor arranged help 
to support him in making friends and improving his health.
Think and Reflect: What was the cause of the boy’ s health problems? 
How did his habits and surroundings affect his well-being?
As per the World Health Organization (WHO), health is 
defined as a ‘state of complete physical, mental, and social 
well-being, and not merely the absence of disease’ (Fig. 3.1). A 
healthy person can perform various tasks more efficiently and 
cope well in different and difficult situations. A healthy person 
can adjust well with peer groups and other members of society. 
Let us understand more about health.
Ayurveda teaches us that true health is a balance of body, mind, and 
surroundings.
Following dinacharya (daily routine) and 
ritucharya (seasonal routine) helps maintain this 
balance. Eating fresh, wholesome food suited to 
one’s prakriti (body constitution) is essential. 
Regular exercise, cleanliness, restful sleep, and 
a calm mind support overall well-being. This 
can also be achieved through practices like yoga, 
meditation, and mindfulness.
Our scientific heritage
3.2 How Can We Stay Healthy?
Staying healthy means eating nutritious food, maintaining 
hygiene, staying in a clean place, exercising regularly, getting 
Fig. 3.1: Aspects of 
health
Mental
Physical Social
Chapter 3.indd   29 Chapter 3.indd   29 6/28/2025   5:56:36 PM 6/28/2025   5:56:36 PM
Page 3


28
Curiosity — Textbook of Science for Grade 8
Immersion into social media is 
spurring rise in loneliness
11.4% of India’s population is 
diabetic 
Fast food is one of the leading causes of 
diabetics 
Air pollution is one of the leading 
risk factors for health
Excessive screen watching leads to dry eye 
disease in children  
More than 100 million people 
in India are diabetic: ICMR report
Yoga reduces anxiety and 
increase concentration
Air pollution is a public 
health emergency
Increase in screen time in children leads to 
a rise in obesity, sleep disorders, and anxiety 
28.6% Indians are obese
Arrival of monsoon leads to increase in 
malaria and engue cases
Diet mantra: Reduce the amount 
of oil and sugar in diet
Regular handwashing reduces 
infection by 50%
Diabetes & heart attack are more 
prevalent in people of South-east 
Asia: WHO 
Safe drinking water reduces cases of water 
and vector-borne diseases 
FIT INDIA MOVEMENT
Reduce oil in diet by 10%
Yoga for the mind, body and soul
 
28 Curiosity — Science Textbook for Grade 8
3
Health: 
The Ultimate Treasure
Probe and ponder
 z How does your body respond to an infection such as common cold? 
 z We rarely see cases of smallpox or polio these days, but diseases 
like diabetes and heart problems are more common. Why?
 z Could climate change lead to new types of diseases?
 z How do emotions like stress or worry affect us and make us sick?   
 z Why do some groups of people get affected more than others 
during disease outbreaks?
 z Share your questions
 
 ? 
Chapter 3.indd   28 Chapter 3.indd   28 6/28/2025   5:56:27 PM 6/28/2025   5:56:27 PM
Chapter 3?—?Health: The Ultimate Treasure 29
3.1 Health: Is It More Than Not Falling Sick?
What do the news clippings on the notice board tell you about 
people’s health in our country? Is being healthy just about not 
having diseases? Health also includes feeling good physically, 
staying positive, and having strong relationships. A healthy person 
takes care of their body, maintains a positive mindset, and enjoys 
social life. Let us now explore what it truly means to be healthy.
Activity 3.1:  Let us read
A Grade 8 student moved to a new school in another city. With no 
friends in his new environment and busy parents, he felt lonely. To 
cope, he spent more time on his phone and social media, but this made 
him feel worse. He stopped trying to make friends, had headaches, 
lost weight, and could not sleep well. A doctor advised less screen 
time and meeting a counsellor. The school counsellor arranged help 
to support him in making friends and improving his health.
Think and Reflect: What was the cause of the boy’ s health problems? 
How did his habits and surroundings affect his well-being?
As per the World Health Organization (WHO), health is 
defined as a ‘state of complete physical, mental, and social 
well-being, and not merely the absence of disease’ (Fig. 3.1). A 
healthy person can perform various tasks more efficiently and 
cope well in different and difficult situations. A healthy person 
can adjust well with peer groups and other members of society. 
Let us understand more about health.
Ayurveda teaches us that true health is a balance of body, mind, and 
surroundings.
Following dinacharya (daily routine) and 
ritucharya (seasonal routine) helps maintain this 
balance. Eating fresh, wholesome food suited to 
one’s prakriti (body constitution) is essential. 
Regular exercise, cleanliness, restful sleep, and 
a calm mind support overall well-being. This 
can also be achieved through practices like yoga, 
meditation, and mindfulness.
Our scientific heritage
3.2 How Can We Stay Healthy?
Staying healthy means eating nutritious food, maintaining 
hygiene, staying in a clean place, exercising regularly, getting 
Fig. 3.1: Aspects of 
health
Mental
Physical Social
Chapter 3.indd   29 Chapter 3.indd   29 6/28/2025   5:56:36 PM 6/28/2025   5:56:36 PM
30
Curiosity — Textbook of Science for Grade 8
proper sleep, spending time with family and friends, and having 
a positive attitude. What we should do and what should not do to 
keep ourselves healthy?
Activity 3.2:  Let us list
 z List some good habits that your parents, teachers, or elders 
often encourage you to follow. How many of these are already 
a part of your daily routine? Which ones would you like to 
start following? Add to the list below:
 { Keep yourself clean and maintain personal hygiene.
 { Eat a healthy and balanced diet.
 { Exercise regularly.
 { Make time to relax or meditate every day.
 z Now, think about habits that are not good for your health. 
Add more to the list below:
 { Spending too much time on mobile phones or other digital 
screens.
 { Eating fast food and other junk food every day.
 { Sleeping very late or not getting enough sleep.
 { Skipping meals, especially breakfast.
Taking care of our body and mind is important. Healthy habits 
support a healthy body as well as a healthy mind. 
Discuss your findings with your friends and teacher. From 
the activity you participated in and the discussions, you may 
have realised that our health depends on many factors. These 
factors include our lifestyle (how we live) and our environment 
(our surroundings).
3.2.1 Maintain a healthy lifestyle
 z Eat a balanced diet with plenty of fruits, vegetables, and 
whole grains.
 z Avoid processed, fatty, or sugary food and drinks.
 z Stay physically active by playing outdoors, walking, running, 
cycling, or exercising.
 z Limit screen time and spend more time in nature.  
 z Get enough sleep to help your body and mind rest and recover.
 z Practice yoga or simple breathing exercises like pranayama 
regularly.
 z Say ‘NO’ to harmful substances things like tobacco, alcohol, 
and addictive drugs (Fig. 3.2).
Eat a balanced diet
Stay physically active
Say no to smoking or 
alcohol
Manage stress
Get enough sleep
Fig. 3.2: How to be healthy
Chapter 3.indd   30 Chapter 3.indd   30 6/28/2025   5:56:39 PM 6/28/2025   5:56:39 PM
Page 4


28
Curiosity — Textbook of Science for Grade 8
Immersion into social media is 
spurring rise in loneliness
11.4% of India’s population is 
diabetic 
Fast food is one of the leading causes of 
diabetics 
Air pollution is one of the leading 
risk factors for health
Excessive screen watching leads to dry eye 
disease in children  
More than 100 million people 
in India are diabetic: ICMR report
Yoga reduces anxiety and 
increase concentration
Air pollution is a public 
health emergency
Increase in screen time in children leads to 
a rise in obesity, sleep disorders, and anxiety 
28.6% Indians are obese
Arrival of monsoon leads to increase in 
malaria and engue cases
Diet mantra: Reduce the amount 
of oil and sugar in diet
Regular handwashing reduces 
infection by 50%
Diabetes & heart attack are more 
prevalent in people of South-east 
Asia: WHO 
Safe drinking water reduces cases of water 
and vector-borne diseases 
FIT INDIA MOVEMENT
Reduce oil in diet by 10%
Yoga for the mind, body and soul
 
28 Curiosity — Science Textbook for Grade 8
3
Health: 
The Ultimate Treasure
Probe and ponder
 z How does your body respond to an infection such as common cold? 
 z We rarely see cases of smallpox or polio these days, but diseases 
like diabetes and heart problems are more common. Why?
 z Could climate change lead to new types of diseases?
 z How do emotions like stress or worry affect us and make us sick?   
 z Why do some groups of people get affected more than others 
during disease outbreaks?
 z Share your questions
 
 ? 
Chapter 3.indd   28 Chapter 3.indd   28 6/28/2025   5:56:27 PM 6/28/2025   5:56:27 PM
Chapter 3?—?Health: The Ultimate Treasure 29
3.1 Health: Is It More Than Not Falling Sick?
What do the news clippings on the notice board tell you about 
people’s health in our country? Is being healthy just about not 
having diseases? Health also includes feeling good physically, 
staying positive, and having strong relationships. A healthy person 
takes care of their body, maintains a positive mindset, and enjoys 
social life. Let us now explore what it truly means to be healthy.
Activity 3.1:  Let us read
A Grade 8 student moved to a new school in another city. With no 
friends in his new environment and busy parents, he felt lonely. To 
cope, he spent more time on his phone and social media, but this made 
him feel worse. He stopped trying to make friends, had headaches, 
lost weight, and could not sleep well. A doctor advised less screen 
time and meeting a counsellor. The school counsellor arranged help 
to support him in making friends and improving his health.
Think and Reflect: What was the cause of the boy’ s health problems? 
How did his habits and surroundings affect his well-being?
As per the World Health Organization (WHO), health is 
defined as a ‘state of complete physical, mental, and social 
well-being, and not merely the absence of disease’ (Fig. 3.1). A 
healthy person can perform various tasks more efficiently and 
cope well in different and difficult situations. A healthy person 
can adjust well with peer groups and other members of society. 
Let us understand more about health.
Ayurveda teaches us that true health is a balance of body, mind, and 
surroundings.
Following dinacharya (daily routine) and 
ritucharya (seasonal routine) helps maintain this 
balance. Eating fresh, wholesome food suited to 
one’s prakriti (body constitution) is essential. 
Regular exercise, cleanliness, restful sleep, and 
a calm mind support overall well-being. This 
can also be achieved through practices like yoga, 
meditation, and mindfulness.
Our scientific heritage
3.2 How Can We Stay Healthy?
Staying healthy means eating nutritious food, maintaining 
hygiene, staying in a clean place, exercising regularly, getting 
Fig. 3.1: Aspects of 
health
Mental
Physical Social
Chapter 3.indd   29 Chapter 3.indd   29 6/28/2025   5:56:36 PM 6/28/2025   5:56:36 PM
30
Curiosity — Textbook of Science for Grade 8
proper sleep, spending time with family and friends, and having 
a positive attitude. What we should do and what should not do to 
keep ourselves healthy?
Activity 3.2:  Let us list
 z List some good habits that your parents, teachers, or elders 
often encourage you to follow. How many of these are already 
a part of your daily routine? Which ones would you like to 
start following? Add to the list below:
 { Keep yourself clean and maintain personal hygiene.
 { Eat a healthy and balanced diet.
 { Exercise regularly.
 { Make time to relax or meditate every day.
 z Now, think about habits that are not good for your health. 
Add more to the list below:
 { Spending too much time on mobile phones or other digital 
screens.
 { Eating fast food and other junk food every day.
 { Sleeping very late or not getting enough sleep.
 { Skipping meals, especially breakfast.
Taking care of our body and mind is important. Healthy habits 
support a healthy body as well as a healthy mind. 
Discuss your findings with your friends and teacher. From 
the activity you participated in and the discussions, you may 
have realised that our health depends on many factors. These 
factors include our lifestyle (how we live) and our environment 
(our surroundings).
3.2.1 Maintain a healthy lifestyle
 z Eat a balanced diet with plenty of fruits, vegetables, and 
whole grains.
 z Avoid processed, fatty, or sugary food and drinks.
 z Stay physically active by playing outdoors, walking, running, 
cycling, or exercising.
 z Limit screen time and spend more time in nature.  
 z Get enough sleep to help your body and mind rest and recover.
 z Practice yoga or simple breathing exercises like pranayama 
regularly.
 z Say ‘NO’ to harmful substances things like tobacco, alcohol, 
and addictive drugs (Fig. 3.2).
Eat a balanced diet
Stay physically active
Say no to smoking or 
alcohol
Manage stress
Get enough sleep
Fig. 3.2: How to be healthy
Chapter 3.indd   30 Chapter 3.indd   30 6/28/2025   5:56:39 PM 6/28/2025   5:56:39 PM
Chapter 3?—?Health: The Ultimate Treasure 31
3.2.2 Keep the environment clean
Activity 3.3: Let us compare
 z Look at Fig. 3.3a and Fig. 3.3b.  Which playground would you 
like to play in, and why?  
 z Most of us would like to play in the 
playground shown in Fig. 3.3a as it is clean, 
well-maintained, and looks beautiful. The 
playground in Fig. 3.3b is polluted, dirty, 
unhygienic, and full of flies and mosquitoes. 
People living in such areas may fall sick 
more often. 
 z In addition to inculcating good habits and 
adopting a healthy lifestyle, we must keep 
ourselves and our surroundings clean.  
 z Have you ever found it hard to breathe in 
a place with a lot of smoke or dust? That is 
because clean air and water are important 
for our health. In cities, air pollution from 
vehicles and factories can cause problems 
like coughing or asthma. The Air Quality 
Index (AQI) helps us know how clean the 
air is. A cleaner environment helps us stay 
healthy and feel better.
 z But health is not only about the body. 
Our feelings and relationships matter too. 
Even if we eat well and live in a clean place, 
we may not feel good if we are lonely or upset. Spending 
time with friends and family, talking, laughing, and having 
fun help keep our minds healthy too.
3.3 How Do We Know That We Are Unwell?
Our body usually works in a certain way to keep us healthy. 
When we feel unwell, it means something inside us may not 
be working as it should. We may have symptoms, such as pain, 
tiredness, or dizziness, and signs like fever, rash, high blood 
pressure, or swelling that indicate we are unwell. A symptom 
is what we feel (like pain), while a sign is something that can be 
seen or measured (like high body temperature when we have 
fever). These help doctors understand what might be making us 
unwell.
(a)
(b)
Fig. 3.3: Two different playgrounds 
Chapter 3.indd   31 Chapter 3.indd   31 6/28/2025   5:56:44 PM 6/28/2025   5:56:44 PM
Page 5


28
Curiosity — Textbook of Science for Grade 8
Immersion into social media is 
spurring rise in loneliness
11.4% of India’s population is 
diabetic 
Fast food is one of the leading causes of 
diabetics 
Air pollution is one of the leading 
risk factors for health
Excessive screen watching leads to dry eye 
disease in children  
More than 100 million people 
in India are diabetic: ICMR report
Yoga reduces anxiety and 
increase concentration
Air pollution is a public 
health emergency
Increase in screen time in children leads to 
a rise in obesity, sleep disorders, and anxiety 
28.6% Indians are obese
Arrival of monsoon leads to increase in 
malaria and engue cases
Diet mantra: Reduce the amount 
of oil and sugar in diet
Regular handwashing reduces 
infection by 50%
Diabetes & heart attack are more 
prevalent in people of South-east 
Asia: WHO 
Safe drinking water reduces cases of water 
and vector-borne diseases 
FIT INDIA MOVEMENT
Reduce oil in diet by 10%
Yoga for the mind, body and soul
 
28 Curiosity — Science Textbook for Grade 8
3
Health: 
The Ultimate Treasure
Probe and ponder
 z How does your body respond to an infection such as common cold? 
 z We rarely see cases of smallpox or polio these days, but diseases 
like diabetes and heart problems are more common. Why?
 z Could climate change lead to new types of diseases?
 z How do emotions like stress or worry affect us and make us sick?   
 z Why do some groups of people get affected more than others 
during disease outbreaks?
 z Share your questions
 
 ? 
Chapter 3.indd   28 Chapter 3.indd   28 6/28/2025   5:56:27 PM 6/28/2025   5:56:27 PM
Chapter 3?—?Health: The Ultimate Treasure 29
3.1 Health: Is It More Than Not Falling Sick?
What do the news clippings on the notice board tell you about 
people’s health in our country? Is being healthy just about not 
having diseases? Health also includes feeling good physically, 
staying positive, and having strong relationships. A healthy person 
takes care of their body, maintains a positive mindset, and enjoys 
social life. Let us now explore what it truly means to be healthy.
Activity 3.1:  Let us read
A Grade 8 student moved to a new school in another city. With no 
friends in his new environment and busy parents, he felt lonely. To 
cope, he spent more time on his phone and social media, but this made 
him feel worse. He stopped trying to make friends, had headaches, 
lost weight, and could not sleep well. A doctor advised less screen 
time and meeting a counsellor. The school counsellor arranged help 
to support him in making friends and improving his health.
Think and Reflect: What was the cause of the boy’ s health problems? 
How did his habits and surroundings affect his well-being?
As per the World Health Organization (WHO), health is 
defined as a ‘state of complete physical, mental, and social 
well-being, and not merely the absence of disease’ (Fig. 3.1). A 
healthy person can perform various tasks more efficiently and 
cope well in different and difficult situations. A healthy person 
can adjust well with peer groups and other members of society. 
Let us understand more about health.
Ayurveda teaches us that true health is a balance of body, mind, and 
surroundings.
Following dinacharya (daily routine) and 
ritucharya (seasonal routine) helps maintain this 
balance. Eating fresh, wholesome food suited to 
one’s prakriti (body constitution) is essential. 
Regular exercise, cleanliness, restful sleep, and 
a calm mind support overall well-being. This 
can also be achieved through practices like yoga, 
meditation, and mindfulness.
Our scientific heritage
3.2 How Can We Stay Healthy?
Staying healthy means eating nutritious food, maintaining 
hygiene, staying in a clean place, exercising regularly, getting 
Fig. 3.1: Aspects of 
health
Mental
Physical Social
Chapter 3.indd   29 Chapter 3.indd   29 6/28/2025   5:56:36 PM 6/28/2025   5:56:36 PM
30
Curiosity — Textbook of Science for Grade 8
proper sleep, spending time with family and friends, and having 
a positive attitude. What we should do and what should not do to 
keep ourselves healthy?
Activity 3.2:  Let us list
 z List some good habits that your parents, teachers, or elders 
often encourage you to follow. How many of these are already 
a part of your daily routine? Which ones would you like to 
start following? Add to the list below:
 { Keep yourself clean and maintain personal hygiene.
 { Eat a healthy and balanced diet.
 { Exercise regularly.
 { Make time to relax or meditate every day.
 z Now, think about habits that are not good for your health. 
Add more to the list below:
 { Spending too much time on mobile phones or other digital 
screens.
 { Eating fast food and other junk food every day.
 { Sleeping very late or not getting enough sleep.
 { Skipping meals, especially breakfast.
Taking care of our body and mind is important. Healthy habits 
support a healthy body as well as a healthy mind. 
Discuss your findings with your friends and teacher. From 
the activity you participated in and the discussions, you may 
have realised that our health depends on many factors. These 
factors include our lifestyle (how we live) and our environment 
(our surroundings).
3.2.1 Maintain a healthy lifestyle
 z Eat a balanced diet with plenty of fruits, vegetables, and 
whole grains.
 z Avoid processed, fatty, or sugary food and drinks.
 z Stay physically active by playing outdoors, walking, running, 
cycling, or exercising.
 z Limit screen time and spend more time in nature.  
 z Get enough sleep to help your body and mind rest and recover.
 z Practice yoga or simple breathing exercises like pranayama 
regularly.
 z Say ‘NO’ to harmful substances things like tobacco, alcohol, 
and addictive drugs (Fig. 3.2).
Eat a balanced diet
Stay physically active
Say no to smoking or 
alcohol
Manage stress
Get enough sleep
Fig. 3.2: How to be healthy
Chapter 3.indd   30 Chapter 3.indd   30 6/28/2025   5:56:39 PM 6/28/2025   5:56:39 PM
Chapter 3?—?Health: The Ultimate Treasure 31
3.2.2 Keep the environment clean
Activity 3.3: Let us compare
 z Look at Fig. 3.3a and Fig. 3.3b.  Which playground would you 
like to play in, and why?  
 z Most of us would like to play in the 
playground shown in Fig. 3.3a as it is clean, 
well-maintained, and looks beautiful. The 
playground in Fig. 3.3b is polluted, dirty, 
unhygienic, and full of flies and mosquitoes. 
People living in such areas may fall sick 
more often. 
 z In addition to inculcating good habits and 
adopting a healthy lifestyle, we must keep 
ourselves and our surroundings clean.  
 z Have you ever found it hard to breathe in 
a place with a lot of smoke or dust? That is 
because clean air and water are important 
for our health. In cities, air pollution from 
vehicles and factories can cause problems 
like coughing or asthma. The Air Quality 
Index (AQI) helps us know how clean the 
air is. A cleaner environment helps us stay 
healthy and feel better.
 z But health is not only about the body. 
Our feelings and relationships matter too. 
Even if we eat well and live in a clean place, 
we may not feel good if we are lonely or upset. Spending 
time with friends and family, talking, laughing, and having 
fun help keep our minds healthy too.
3.3 How Do We Know That We Are Unwell?
Our body usually works in a certain way to keep us healthy. 
When we feel unwell, it means something inside us may not 
be working as it should. We may have symptoms, such as pain, 
tiredness, or dizziness, and signs like fever, rash, high blood 
pressure, or swelling that indicate we are unwell. A symptom 
is what we feel (like pain), while a sign is something that can be 
seen or measured (like high body temperature when we have 
fever). These help doctors understand what might be making us 
unwell.
(a)
(b)
Fig. 3.3: Two different playgrounds 
Chapter 3.indd   31 Chapter 3.indd   31 6/28/2025   5:56:44 PM 6/28/2025   5:56:44 PM
32
Curiosity — Textbook of Science for Grade 8
3.4 Diseases: What Are the Causes and Types?
A disease is a condition that affects the normal working of the 
body or mind. It can happen when one or more organs or organ 
systems stop functioning properly. Some diseases are caused by 
germs like bacteria, viruses, fungi, worms, or even by protozoa 
(single-celled organisms). These disease-causing organisms are 
called pathogens. Other diseases may result from poor nutrition 
or an unhealthy lifestyle. Some diseases last for a short time, while 
others can continue for a long time and need regular treatment 
or care. Diseases can be grouped into two major types based on 
their causes and how they spread:
 z Non-communicable 
diseases— Some diseases, like 
cancer, diabetes, or asthma, 
are not caused by pathogens 
and do not spread from one 
person to another. They are 
usually linked to lifestyle, diet, 
and/or environment.
 z Communicable diseases— 
Diseases caused by pathogens 
are called communicable 
diseases. They can spread 
from one person to 
another. Some examples 
of communicable diseases 
are typhoid, dengue, flu, 
chickenpox, and COVID-19. 
In recent years, non-communicable diseases (NCDs) like 
diabetes, heart disease, and cancer have become more common 
in India. This is happening because of changes in how people 
live — such as eating more processed food, getting less exercise, 
and living longer lives. Today, most deaths in India are caused 
by NCDs. Understanding the difference between these two types 
helps us know how diseases spread and how to prevent them.
3.4.1 How are communicable diseases caused and spread?
All communicable diseases are caused by pathogens. These 
pathogens can enter our body through the air we breathe or by 
consuming contaminated food or water and more. But how do 
these pathogens spread from one person to another? One common 
way is through air, when an infected person coughs or sneezes, or 
through direct contact like shaking hands, or indirectly by sharing 
Fig. 3.4: Common methods of transmission of diseases
Infected 
person
Healthy
person
Mosquito insects
By food
Direct contact
Indirect 
contact
By air
Healthy person
Rabid animal
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