CBSE NCERT Notes for Class 9 Science Chapter 13 Why Do We Fall Ill?

Why Do We Fall Ill Class 9 CBSE Notes – Chapter 13

CBSE Class 9 Science notes will assist students in studying the topic thoroughly and clearly.

These CBSE Class 9 Science notes were written by subject experts who made the study material very basic, both in terms of language and format.


A condition of whole physical, mental, and social well-being is defined as health. It is not only the absence of sickness or disability as defined by the WHO (1948). Thus, health implies a sense of well-being on all levels: physical, mental, and social.


The following benefits come from good health:

  • It improves our efficiency. It enables us to do a variety of tasks at our best.
  • It enables us to deal with social and psychological pressures with relative ease.
  • It adds happiness to our lives.


Each individual’s health is determined by his or her personal behaviours, economic situation, and physical environment. Personal health is a term that relates to an individual’s total well-being. However, since human beings live in communities, the health of the community has a factor in an individual’s health.

Community health refers to the preservation, protection, and improvement (of an individual’s total well-being) of the whole community in which he or she lives.

Numerous elements affecting an individual’s health include the following:

  1. Social environment : Our physical environment is determined by our immediate surroundings. The environment in which we reside (village, town, or city) has a significant impact on our individual health.
  2. Public cleanliness : If no agency is in charge of waste collection and disposal, the probability of bad health grows. Open drain water that sits stagnant around us also contributes to health problems. As a result, public sanitation is critical for individual health.
  3. Good economic conditions and jobs : Food that is hygienic may be gained via work. Work opportunities should be offered to facilitate this. This promotes individual health.
  4. Social equality and harmony : To maintain our health, we must be joyful. To do this, we must establish positive relationships with our culture. We should not treat one another badly.

Distinctions between Healthy and Disease-Free 

The term disease, i.e. disturbed ease, refers to being uneasy and uncomfortable. Each disease has a distinct and distinct source of suffering. However, disease-free does not always imply health. The following table summarises the distinctions between the two conditions:

People who are in a state of complete health in all areas of their body, mind, and social life.It is a state in which there is no pain or mismatches of body parts and their functions.
A person who is healthy is energetic and can do what needs to be done.Performance by someone who isn’t sick depends on their environment and how they feel about themselves.
It depends on the person as well as on the physical and social factors in the area where he or she lives (societies and communities).It is only about one person.
A person who is healthy will not get sick.A person who isn’t sick can be healthy or unhealthy.


Disease is defined as any disease that impairs or affects an organism’s normal functioning. Each organ system in our body is comprised of distinct organs that conduct distinct roles.

When a disease is present, the functioning or appearance of one or more of the body’s systems may alter. These alterations result in disease symptoms and signs.

Symptoms : Symptoms of illnesses are signals that something is wrong or odd, such as a cold, a headache, a cough, or irregular digestive movements. These suggest the possibility of a disease but do not specify the nature of the sickness. For instance, a headache is seldom indicative of meningitis; it may be caused by official stress or any other ailment.

Signs : The symptoms of an illness provide more conclusive evidence of the disease’s existence. They assist physicians (physicians) in diagnosing disease.


Diseases appear differently based on a variety of conditions. Diseases may be classified into two groups, depending on the length of infection:

  1. Acute diseases are those that occur for a brief period of time. As a result, they have no long-term adverse impact on human health. Patients heal entirely from these diseases, e.g. common cold, cough, etc.
  2. Chronic diseases are those that persist for an extended period of time, perhaps even a lifetime. They have a detrimental influence on human health over the long run, e.g. elephantiasis (an infection caused by a filarial worm), cancer, and TB (T.B).

Chronic Diseases And Poor Health

Acute and chronic illnesses affect our health in distinct ways.

Any disease that impairs the functioning of one area of the body has an effect on our overall health, since all bodily components must operate correctly. Acute diseases do not have enough time to have a significant impact on overall health. Chronic diseases have a significant impact due to their duration. In comparison to acute disorders, they might bring significant discomfort to the health.

Causes Of Diseases: The majority of diseases will have several causes rather than a single one. These disease-causing agents are categorised into two categories:

  • Immediate Causes : These are the genuine or main reasons, sometimes referred to as the first level of causality.

This category includes a variety of microorganisms such as bacteria, viruses, fungus, and protozoans that are capable of causing infectious diseases.

  • Contributory Causes : Additionally, they are referred to as intrinsic or internal factors. They do not cause disease in and of themselves.
  • These may be classified as follows:
    • Poor health due to inadequate diet : It happens as a result of a nutritional deficit, making the individual sick or prone to illness, such as Kwashiorkor, a nutritional deficiency disease caused by a low protein diet. As a result, malnutrition becomes the secondary cause of disease.
    • Genetic disorders : These are present from birth and are inherited from parents to offspring.
    • Lack of public services or poor economic conditions : Poverty also has a role in the spread of disease. These factors contribute to the disease’s third degree of causation.

Infectious And Non-Infectious Causes

Diseases have two separate immediate causes. Infectious agents such as microorganisms produce one kind of disease. Diseases induced by these microbes also spread as a result of their dissemination.

As a result, these disorders are referred to as infectious diseases. The second classification of diseases is those that are not caused by infectious agents. These diseases are infectious exclusively to those who are afflicted with them.

These are referred to as non-infectious diseases or non-communicable diseases because they do not transfer from an infected person to a healthy one. For instance, several types of cancer are caused by genetic defects or by exposure to carcinogenic chemicals and radiation. High blood pressure may be caused by a variety of factors, including obesity and a lack of exercise.


Infectious diseases are caused by a variety of biological agents (pathogens), including viruses, bacteria, fungi, and single-celled animals (protozoans), among others. These diseases may spread quickly from one person to another by water, air, food, insects (vectors), or direct contact.

Infectious Agents

Infectious agents are unicellular or multicellular organisms that cause infection. These pathogenic organisms are also referred to as pathogens and are divided into a variety of categories. They include viruses, bacteria, fungus, protozoans, and nematodes.

Antibiotics : These are the chemical compounds that are mostly generated by microbes (bacteria and fungi). They have the capacity to inhibit the growth of other bacteria at low concentrations. They often inhibit vital biochemical routes for bacteria, such as penicillin.

The antibiotic penicillin prevents bacteria from forming a cell wall.

This cell wall contributes to bacteria’s defence. Bacteria become unable to form a cell wall as a result of penicillin and perish quickly.

Numerous antibiotics are effective against several types of bacteria rather than just one. Antibiotics are useless against viral infections, since viruses do not follow the same metabolic route as bacteria. As a result, using antibiotics during a common cold has little effect on the severity or duration of the illness.

  • Means of Spread : Because infectious illnesses may be transmitted from an infected person to a healthy person in a variety of ways, they are also referred to as communicable diseases. Diseases transmitted by a variety of pathways, including air, water, sexual contact, and vectors. These are addressed in further detail below.
  • Airborne Diseases : The pathogenic microbes that cause these diseases spread through the air. When an infected individual sneezes or coughs, little droplets are expelled. These droplets may be breathed by someone standing nearby, giving the bacteria an opportunity to spread. For instance, the common cold, the common cough, pneumonia.

Airborne diseases are easier to get when we are in close vicinity to the infected individual. The droplet nuclei recirculate in enclosed environments, posing a danger to everyone. Housing that is overcrowded and poorly ventilated contributes significantly to the spread of airborne diseases.

  • Waterborne Diseases : Microbes spread these diseases through water. When the excreta of someone who has an infectious gut disease, such as cholera, is combined with the drinking water used by neighbouring residents, waterborne sickness occurs. In the absence of a safe source of drinking water, such diseases are considerably more prone to spread. For example, cholera-causing microbes may infect new hosts through the water they consume and cause disease.
  • Sexually Transmitted Diseases (STDs) : These are microbiological diseases, and therefore may be spread sexually. STDs, on the other hand, do not spread by casual physical contact. It includes hugs, handshakes, sports such as wrestling, and any other manner humans interact socially. Sexually transmitted diseases such as syphilis and AIDS are common.
  • Spread of Diseases through Vectors : Numerous animals that live closest to humans in the environment are capable of transmitting diseases (germs) from a sick person to a healthy one (another potential host). These animals serve as an intermediary and are referred to be vectors (carriers of disease or infection), e.g.

Mosquitoes such as – Anopheles (female)—malaria, Cu/ex—filariasis, Aedes—yellow fever, dengue. 

Flies such as – housefly—typhoid, cholera, etc., tse-tse fly—sleeping sickness, sandfly—kala-azar. 


Our immune system is important for health. It is the body’s protective mechanism against numerous sorts of microorganisms. Additionally, it protects the body by eradicating infectious microorganisms.

When infectious bacteria enter the body, the immune system responds by activating. A healthy immune system attracts a large number of cells to the damaged region and eliminates disease-causing microorganisms. This process of recruitment is referred to as inflammation. Local consequences such as swelling and discomfort may occur as a result of this process, as may systemic effects such as fever.

Allergies (heightened sensitivity to a foreign substance that causes the body’s defence system to overreact) may sometimes emerge as a result of allergens.

In some cases, the infection’s tissue specificity results in seeming systemic consequences. HIV, for example, infects the immune system and impairs its function. As a result, the body is unable to fight off even small diseases such as a cold or diarrhoea. This might result in death.

Indirectly, the severity of disease symptoms is related to the quantity of microbes (pathogens) in the body. When the number of microorganisms is very low, disease signs may be subtle or invisible. However, if the number of germs is high, the disease may become severe enough to be fatal. The immune system’s importance is readily apparent from the fact that it plays a significant role in determining the number of microorganisms that survive in the body.


Our body is huge in contrast to bacteria. As a result, they may enter the body through a variety of channels and impact specific tissues or organs. The microbe’s residence is chosen in relation to their place of arrival, e.g.

  1. They are more likely to reach the lungs if they enter from the air through the nose. This is the mechanism through which the tuberculosis-causing bacteria enters the body.
  2. If they enter via the mouth, they may remain in the gut lining, such as bacteria that cause typhoid. They may also spread to the liver, similar to the virus that causes jaundice. 
  3. If they enter via sexual contact, they can spread to lymph nodes throughout the body, similar to HIV.
  4. If they enter through mosquito bites, they may spread to the liver and subsequently to Red Blood Cells (RBCs), where they may replicate and cause malaria. They have the potential to enter the brain, similar to the Japanese encephalitis virus, which causes brain fever.

Thus, the signs and symptoms of a disease will vary according to the tissue or organ that bacteria are targeting, e.g.

  1. If the lungs are targeted, symptoms include coughing and shortness of breath.
  2. If the liver is the target organ, jaundice may ensue.
  3. If the brain is the target, symptoms such as headache, vomiting, fits, or loss of consciousness may ensue.


Along with these organ- and tissue-specific impacts of infectious diseases, there are other general consequences that are dependent on the body’s immune system.


When we have an infectious condition, our doctor may prescribe a variety of different medications to treat it.

The following are two approaches of infectious disease treatment:

  • To Reduce The Disease’s Consequences 
    • By delivering a targeted treatment, symptoms may be reduced.
    • Typically, the symptoms are caused by inflammation. For instance, we may use medications to lessen fever, discomfort, or slow movements, or we might rest completely to preserve energy. However, it will not eliminate the harmful microorganisms. As a result, disease would not be totally healed.
  • To Kill The Cause Of The Disease
    • To eliminate the disease’s source Taking medication that destroys bacteria is the most effective strategy to heal completely from a particular disease. It is critical to take medication that is particular to the disease-causing microorganism. Antibiotics may be used to cure bacterial disease, for example, by blocking critical metabolic processes in bacteria (which may include mechanisms for the synthesis of new chemicals or respiration) without affecting our own. 
    • Likewise, there are medications that are capable of killing protozoans, such as the malarial parasite. Antiviral drugs are more difficult to develop since viruses possess relatively few biochemical pathways of their own.

      They get access to our cells and use our machinery to carry out their life activities. This results in a dearth of virus-specific targets. However, effective antiviral medications, such as the one that controls HIV infection, are now accessible.


We now have antibiotics and other medications to treat a wide variety of diseases as a result of modern technology and scientific advances.

However, there are a few restrictions that are often faced while treating infectious diseases.

The following are some of the limits or disadvantages of infectious diseases:

  1. Once a person is identified with a disease, his or her biological functions are impaired and may never fully recover; 
  2. Treatment will take time. This suggests that someone suffering from a disease is likely to stay bedridden for an extended period of time, even with good treatment. 
  3. A person who is ill with an infectious disease may act as a source of infection for other persons. As a result, the issues outlined before are multiplied.


There are two major methods to disease prevention. These are as follows:

General Ways of Preventing Infectious Diseases : These methods are primarily concerned with avoiding exposure to infectious microorganisms. Public hygiene is also crucial for disease prevention.

  • Strong immune system : Normally, the immune system of our body fights and destroys microbes that enter our body. If the immune system is functioning properly, we do not get any disease. The immune system’s cells eliminate microbes before they reach epidemic proportions. 

As is well known, when the number of injecting microorganisms is low, disease signs are minimal. In other words, exposure to or infection with an infectious microbe does not always result in the development of visible disease.

  • Balanced diet : The immune system will not operate correctly if there is insufficient food and nutrients available. As a result, the second fundamental premise of prevention is the provision of enough and nutritious food for everyone.

Specific Ways of Preventing Infectious Diseases  : It entails the adoption of immunisation practises.


When the body detects an infectious microorganism for the first time, it mounts a defence against it and then recalls it precisely. Thus, the next time that specific microbe or one of its close cousins enters the body. The immune system reacts more vigorously. It is more effective in eliminating infection than the first time I used it. This is the fundamental premise of vaccination.


Although there is no smallpox in the world now, outbreaks of this disease were widespread a century ago.

However, there was one group of individuals who offered nursing care to disease patients and were unafraid of contracting the ailment. This was a group of people who had previously suffered smallpox and survived. Thus, contracting the disease just once protects you against future episodes of the same disease. This is related to the immune system’s memory capacity.


  1. This phrase is derived from the Latin word Vaccinia, which means cowpox, and Vacca, which means cow. It is the technique of injecting dead germs into the body of a healthy individual in order to establish immunity against that microbe’s specific disease.
  2. This effectively removes the infection because when the immune system encounters an infectious germ for the first time, it reacts to it and then remembers it uniquely. Thus, the body builds a memory of the specific disease by introducing a substance into the body that mimics the microorganisms against which we want to vaccinate.
  3. Vaccines are preparations of infectious pathogens that have been weakened or destroyed, or their products. They induce the immune system to create antibodies directed against a certain disease. Edward Jenner developed the first vaccination against smallpox.
  4. Numerous vaccinations exist to prevent infectious diseases and give disease-specific protection.

The following are the diseases for which vaccinations are available:

  • Diphtheria, Pertussis, Tetanus — DPT (Hib vaccine) 
  • Poliomyelitis — OPV (Oral Polio Vaccine) 
  • Hepatitis-B — Hepatitis vaccine 
  • Tuberculosis — BCG (Bacillus Calmette Guerin)

These vaccinations are accessible via the child immunisation programme. Vaccines must be administered at the appropriate age; for example, a vaccination against hepatitis-A is commercially available. The hepatitis virus causes jaundice and is spread through water. It is ineffective if the vaccination is administered beyond the age of five since by that time, the child has been exposed to a disease (virus through water) and has developed immunity against it. This is because they get infected by water.

Pulse Polio Programme

In 1995, India began the Pulse Polio Immunisation (PPI) campaign in response to the World Health Organization’s (WHO) Global Polio Eradication Initiative.

All children under the age of five get two doses of Oral Polio Vaccine (OPV) in December and January of each year until polio is eliminated. PPI was established with the goal of reaching 100 percent coverage. India has been declared polio-free by the World Health Organization. However, the pulse polio campaign is systematically eradicating any possibility of a return.

NCERT questions & answers from Why Do We Fall Ill?

Question 1.

State any two conditions essential for good health.

  • Complete physical fitness free from any disease.
  • Perfect mental (and social) well being.

Answer: State any two conditions essential for being free of disease.

  1. Living circumstances that are not overcrowded, safe drinking water, and a clean atmosphere.
  2. A balanced diet, personal cleanliness, exercise, and relaxation are all necessary components.

Question 2.

Are the answers to the above questions same or different ? Why ?


Different: While health is a condition of complete physical, mental, and social well-being, sickness is a state of disease or discomfort in the body or a part of the body.

Question 3.

List any three reasons why you would think that you are sick and ought to see a doctor. If only one of these symptoms were present, would you still go to the doctor ? Why or why not ?


  1. Cough, fever, and cold. The illness must be caused by an infection, which requires correct diagnosis and treatment, which can only be provided by a physician.
  2. Even if only one symptom is present (for example, a cough or a cold), I prefer to see a doctor because an untreated illness might progress and do additional damage to the body.

Question 4.

In which of the following case do you think the long term effects on your health are likely to be most unpleasant ?


(a) If you get jaundice

(b) If you get lice

(c) If you get acne.

Jaundice. It is a severe infection that requires several days to heal. During this time span, bile pigments accumulate in various locations throughout the body, permanently damaging several of them.

Question 5.

Why are we normally advised to take bland and nourishing food when we are sick ? (CCE 2011, 2012)

Answer: It aids in the strengthening of the immune system and replenishes nutrients lost by pathogenic pathogens.

Question 6.

What are the different means by which infectious diseases are spread ?

Answer: Directly from the air or as droplets, e.g., cold.

By consuming infected food and water, for example, cholera.

By fomites or materials contaminated with the patient’s faeces, for example, chickenpox.

Question 7.

Contagious diseases simply spread by contact.

Answer: In the case of AIDS, syphilis, and some other diseases, through sexual and blood contact.

By vectors and carriers, for example, malaria is transmitted by female Anopheles.

Question 8.

What precautions would you take in your school to reduce incidence of infectious diseases ?


  1. Classrooms that are open and well-proportioned.
  2. Advise students who are ill not to attend school. If such a pupil does arrive, he or she should be given a separate bench.
  3. Assuring the safety of drinking water
  4. Cleanliness.
  5. Anti-fly and anti-mosquito protection.
  6. Vaccination against communicable diseases.
  7. Students must get routine medical examinations.

Question 9.

What is immunisation?

Answer: Immunization is the process of developing immunity or resistance to a pathogen by vaccination.

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