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Why Do We Fall Ill Class 9 Notes CBSE Science Chapter 13 (Free PDF Download)

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Last updated date: 09th Apr 2024
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CBSE Class 9 Science Chapter 13 - Why Do We Fall ill Revision Notes - Free PDF Download

Health is a state of complete mental, social, and physical well-being and not just the absence of any disease or infirmity. A healthy person is not just physically fit but also mentally agile. The health of an individual is directly influenced by the environment in which he lives. Therefore, one should take care of his surroundings and make it pollution-free to stay healthy. 

But, most importantly, food, as explained in chapter 13 science class 9 notes, is crucial for our growth and development. Vedantu’s Why Do We Fall Ill class 9 notes tell us the reasons for our illness and how they are all related to our diet or environment or both. 

Vedantu is a platform that provides free CBSE Solutions (NCERT) and other study materials for students. You can also download Class 9 Maths and Class 9 Science NCERT Solutions to help you to revise complete syllabus and score more marks in your examinations.

Download CBSE Class 9 Science Revision Notes 2023-24 PDF

Also, check CBSE Class 9 Science revision notes for All chapters:


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Access Class 9 Science Chapter 13 – Why do we fall Ill Notes

1. What is Health? What is Its Significance?

i. Health: A state of complete physical, mental and social well-being of a person. Anything that leads to the disturbance with the proper functioning of cells and tissues will cause a lack of proper functioning of the body. Personal as well as community issues both play important roles in health.

ii. The health of all organisms does rely either on their surroundings or their environment. The environment includes the physical environment. Our social environment, public cleanliness, good financial conditions and jobs, social equality, and agreement are also significant in determining an individual‘s health.


2. What do you Mean by “Health” and “Disease Free”?

Disease simply means "uncomfortable". A person suffering with disease will have improper functioning of the organ systems inside. Healthy state is a state where a person is fit as well as fine from all aspects which include physical, mental, psychological, and emotional state. And Healthy means to be free from any kind of disease. But, being free from disease does not mean to be healthy, i.e. a person who is disease-free might not be stable from the mental state as that of a healthy person.


3. Disease and Its Causes

When a person encounters disease, either his functioning or the appearance of the systems of the body will start changes for the worse. These changes will lead to symptoms and signs of that disease. Symptoms of disease are said to be the unusual changes felt by the person. Different symptoms which could occur during the diseased condition include headache, cough ,diarrhea and wounds filled with pus. These symptoms are an indication of a disease, but they don’t indicate the cause of the disease.


4. Acute and Chronic Diseases 

Diseases are classified as acute and chronic diseases on the basis of their duration. Acute diseases usually last for a short period of time without posing adverse effects on health. For example, the common cold. Chronic diseases are those that can last for a prolonged period and can lead to drastic effects on health. Diabetes is one such example of a chronic disease. Chronic diseases tend to have more drastic long-term effects on a person’s health in comparison to acute diseases.


5. Cause of Diseases

Most diseases can have many causes, rather than just one single cause which could be poor nourishment, genetic defects, lack of public services, and by microorganisms. 


6. Infectious and Non-Infectious:

A disease could be Infectious or non-infectious diseases based on the occurrence or spread and duration. Infectious diseases are caused by microorganisms like bacteria, viruses, fungi, and protozoa. These can be transmitted i.e. spread from one person to another through various mediums like air, food, water, etc. Non-infectious diseases are due to genetic abnormalities. They are also known as non-communicable diseases since they do not spread from one person to another. Cancer is one such example of non-infectious disease.


7. Infectious Agents

Disease causing organisms are known as infectious agents. They obtain their nourishment from the host organism. eBacteria, Viruses, Fungi, Protozoa, Helminths are some of its examples. Typhoid, cholera, tuberculosis, anthrax, pimples and peptic   are some of the diseases caused by bacteria and ulcers. Some viral diseases are common cold, influenza, dengue fever, SARS and AIDS. Skin infections are caused by Fungi. Protozoa is responsible for malaria, kala – azar and sleeping sickness. Helminthic worms can cause intestinal infections or elephantiasis.


8. Mode of Transmission:

Infectious diseases are spread through microbial organisms from an infected person when he comes in contact with a healthy person. Hence, they are also called communicable diseases. Microorganisms are also transmitted through physical contact, air, water and vectors.

i. Air Borne Diseases: Diseases that are transmitted through air in the form of droplets. For example, common cold, pneumonia and tuberculosis.

ii. Waterborne Diseases: Diseases transmitted through the excreta from someone suffering from an infectious disease. For example, cholera get mixed with the drinking water used by people living nearby.

iii. Physical Contact: Diseases transmitted through direct contact. AIDS is transmitted through various pathways like blood-blood contact with infected people or from an infected mother to her child during pregnancy, or by intimate contact between partners.

iv. Vectors: Commonly known as carriers. Vectors responsible for transfer of microorganisms from one human being to another are female mosquitoes, dogs, hens, etc. Rabies virus is transmitted by dogs and cats bite(saliva).


9. Organ-Specific and Tissue-Specific Manifestations:

i. Microorganisms target a specific site in the body. Symptoms are indications of the organ infected. Malaria-causing microbes, enters in the human body through a mosquito bite, and reaches the liver, and starts destroying red blood cells.

ii. The signs and symptoms of a disease depend on the tissue or organ that microbe targets. If the liver is targeted, it will lead to jaundice. If the brain is targeted, then headaches, vomiting, fits or unconsciousness will occur.

iii. An active immune system gives instructions to many cells to reach the affected tissue and kill the disease-causing microorganism. This recruitment process is known as inflammation. The immune system plays a major role in determining the number of microbes surviving inside the body.


10. Principles of Treatment and Prevention: There are several ways to treat an infectious disease:

i. First, to reduce the effects of the disease and the second to kill the cause of the disease. Antibiotics drugs are used in treating the diseases caused by bacteria, fungi and protozoans. Penicillin is an antibiotic drug. Antiviral drugs are the medicines used to kill diseases caused by viruses. These drugs are quite difficult to prepare in comparison to antibacterial drugs. Despite this limitation, there are now effective anti-viral drugs in the market like the drugs to keep HIV infection under control.

ii. The general ways of preventing any infection is mainly to prevent infections. Like airborne microorganisms can be prevented by improvement in living conditions which are away from overcrowded places. Water-borne diseases can be prevented by safe drinking water and a sanitized environment. Vector-borne diseases by keeping a clean environment. Public hygiene is one such basic step in order to prevent infectious diseases. Proper and sufficient food can also prevent infectious diseases. Immunization must be done in a timely manner. Vaccines are a specific way of preventing diseases and are helpful against many diseases like vaccines against tetanus, diphtheria, whooping cough, measles, and polio.


Class 9 Science Chapter 13 Notes: Free PDF Download

Notes of Chapter Why Do We Fall lll

Any condition which causes pain, discomfort, abnormal behaviour can lead to a condition known as disease. People can fall ill because of many reasons that can be internal or external. Listed below are some of the reasons why we fall ill:

  • When people consume contaminated or polluted food and water.

  • When any natural flora travels from its native place and resides in another part of the body

  • Due to a weak immune system

  • Change in the environment 

  • When polluted air is inhaled

  • A person’s mental health also affects his or her health

  • Lack of nutrition in the body due to poor diet.


Class 9 Chapter 13 Science Notes: Diseases and Its Types

While our body functions normally, how can we know that we are not well? We can feel something wrong in our body. The organism has several organ systems that contain tissues. Tissues have cells that help our body to perform several activities. When we have any disease, the body starts to function improperly which gives rise to a series of symptoms such as cough, fever, flu, headaches, and many others. This is the time when one should see a doctor. Class 9 science ch 13 notes also tell us that disease can be of two types chronic disease and acute disease.

a) Chronic Diseases: 

Chronic diseases refer to severe diseases that last for a long period and in some cases have a lifelong impact on the patient’s body. These diseases are sometimes not curable. 

  • For example, cancer, asthma, diabetes, arthritis, and many more. The reason for such a disease can be due to a poor lifestyle or hereditary. 

b) Acute Diseases: 

These are temporary diseases which last for a few days and in some cases few weeks. They can be cured and generally do not have a lifelong impact on the patient. For example, typhoid, food poisoning, chickenpox, and so on.

All these are further detailed in the science ch 13 class 9 notes.


Notes of Ch 13 Science Class 9: Different Types of Causes of Disease

Diseases are caused because of pathogens such as bacteria and viruses. Genetic factors and internal factors also are some reasons that cause disease.

a) Infectious Disease

Diseases caused by pathogens and that can spread to the surroundings are called infectious diseases.

b) Non-infectious Diseases

Diseases that do not spread from one another are called non-infectious diseases. They are generally not caused by pathogens.

c) Pathogen

According to notes for class 9 science chapter 13 the pathogen is known as a biological agent that causes illness or disease to its host. Pathogens include harmful microorganisms like viruses, bacteria, protozoa, and fungi. 


Science Chapter 13 Class 9 Notes: Prevention of Disease 

Antibiotics are antimicrobial drugs that are produced from several other organisms like bacteria and fungus and are used for treating others from the harmful infections that are caused by harmful microorganisms or pathogens. Antibiotics function through:

1. Alteration of cell membranes

2. Inhibition of antimetabolite activity.

3. Inhibition of nucleic acid synthesis.

4. Inhibiting of cell wall synthesis.

5. Inhibition of protein synthesis.


Class 9 Science Why Do We Fall Ill Notes: Preventive Measures

Several preventive measures can be taken to avoid infections. One should always maintain hygiene to avoid diseases. People should drink water that is safe and filtered. Immunisation is one of these measures. Immunization refers to the process where an individual is made resistant or immune to a specific infectious disease. The body develops its resistance from the vaccines. 


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FAQs on Why Do We Fall Ill Class 9 Notes CBSE Science Chapter 13 (Free PDF Download)

1. What are the conditions essential for good health?

The two basic reasons for good health are: 

  • Good earnings and overall fair economic condition

  • A conducive social environment in which the basic needs are met

2. What are the different ways by which infections spread?

Infectious diseases mostly spread through the following modes – air, water, body fluids and sexual contacts (cough droplets, semen etc), pests and contact with an affected person. Through these vectors or modes of contact, the disease-causing germs are transmitted from one person to another.

3. What is ‘immunization?’

The method by which our body is naturally or artificially provided with the trigger(s) to generate antibodies against a particular disease or group of diseases is called immunization. Grossly, there are two methods of immunization- natural immunization and vaccine-induced immunization.