CBSE Notes Class 9 Science - Why do we fall ill?

The World Health Organization [W.H.O.] gave the following definition of health in 1948. Health is a state of complete physical, mental and social well being and not merely the absence of disease or infirmity. In 1978 this definition was updated by including- the ability to lead a "Socially and economically productive life". Personal health is a state of complete physical, mental and social well being. Community health comprises of maintaining, improving and protecting the health of entire community. 

1.0Basic Conditions for Good Health

The basic conditions that help in maintaining an individual and community health are : (i) Balanced diet. (ii) Maintaining proper hygienic and sanitary conditions in the environment. (iii) Providing good socio-economic conditions. (iv) Clean food, water and air.(v) Providing health care services. (vi) Imparting health education and promoting public awareness. (vii) Providing proper facilities for preventing diseases. 

Differences Between Disease Free and Healthy

Disease 

• The word disease (dis-ease) literally means disturbed ease or uncomfortness. 
• Thus disease can be defined as "Malfunctioning of the body or a part of it due to one reason or the other". 
• Disease is any harmful change that interferes with the normal appearance, structure or function of the body or any of its parts.

Symptoms

• A symptom is a change in normal functioning of the body which is felt by the patient but cannot be observed by the doctor. 
• Symptoms only indicate that a patient is suffering from a disease, but they don't actually detect the disease.
• Examples: Stomachache, headache. 

Signs:

• A sign of a disease is a change in the body function or structure that can be observed. Signs give doctors a good idea of a disease.
• Examples: Rashes, swelling of gland, change in color of skin.

2.0Types of Disease - on the basis of duration

Disease and its Cause:

In general terms there can be two causes of a disease: 

• Immediate cause- Infection by microorganisms which actually causes the disease.  
• Contributory cause - Lack of nourishment, economic status of household and genetic factors.
• But on a broader scale, human health is affected by various factors ,causes or sources. These factors can be of two major types. 
• Intrinsic or internal factors : Factors inside the body or within the body. E.g. diabetes due to malfunctioning of pancreas. 
• External or extrinsic factors : Factors outside the body. E.g. diet deficiency, disease causing organisms.

3.0Types of diseases 

• Congenital disease : Caused either due to genetic disorders or environmental factors or hereditary diseases. e.g. Hemophilia, sickle cell anemia, diabetes etc. 
• Acquired disease : Acquired by organisms after birth, not heritable. 
• Communicable disease : The infectious agents ( microbes) can move from an affected person to a healthy person, so the infectious diseases are also known as communicable diseases. 
• Non-communicable disease or Non infectious disease : A disease that is not caused by an infectious agent and which does not spread from person to person. e.g. Diabetes, cancer. 
• Degenerative disease : Disease of old age due to gradual decline in functioning of body organs. 
• Deficiency disease : Disease caused by deficiency of one or more nutrients in diet. e.g. Night Blindness (Vitamin-A), Marasmus (PEM), Anaemia (iron). 
• Allergies : Caused due to hypersensitivity of the body to any foreign substance. e.g. Asthma. 
• Cancer : Uncontrolled division of cells in any part of the body to form mass of cells-tumor.

Infectious diseases 

• A disease caused by an infectious agent is called an infectious disease. 
• Infection : The entry and multiplication of a disease-causing organism in the body. 
• Infectious agents (Pathogens) - They are disease- causing organisms, including bacteria, viruses, fungi, protozoan parasites and worms. 
• Host : The organism in which an infectious agent lives and multiplies. 
• Incubation period : Period between the entry of germs and appearance of the first symptom of the disease.

Infectious Agents and Some Diseases Caused by them:

Means of Spread of Infectious Diseases:

Organ specific and Tissue specific Manifestations:

Common Effect of Infectious Disease

• The body immune system is activated in response to an infection. 
• An activated immune system will send white blood cells to the affected tissue to kill the pathogens. 
• This defensive reaction of the body's immune system in response to infection or injury is called inflammation. 
• Local effects of inflammation - swelling, pain and redness. 
• General effects of inflammation - fever. 
• Sometimes an infectious disease might affect the body in such a way that wide-ranging effects are produced.

4.0Principles of Treatment

Symptom Directed Treatment

• Does not cure the disease itself, it is done for the comfort of the patient. 
• Use of medicines to bring down fever, to reduce pain or for loose motions. Bed rest to conserve energy.

Treatment that aims at killing the disease causing microbe

• Usually medicines are used to kill the microbes. 
• It aims at directing or blocking the biochemical pathways used by these pathogens. 
• However care should be taken that these medicines should not affect biochemical pathways important for the body. 
• e.g. Antibiotics, drugs that kill protozoans etc. 

Limitations to Principles of Treatment of Infectious Disease

• The body functions may be damaged due to the disease and the patient may never recover completely.
• Treatment takes time and may cost a lot of money and the patient will be bedridden for some time. 
• During infection our body has a large number of infectious agents multiplying within us. In this state we can be a source of infection and spread it to other people.

Prevention of Infectious diseases 

• Means to stop a disease from occuring by taking certain precautions is called prevention. Two types of methods are used :  
• General methods 
• Methods specific to a disease

General Methods

Preventing exposure to disease or microbe through following ways. 

• Safe drinking water : Filtering to remove suspended particles and treated to kill bacteria. 
• Proper disposal of waste: Covered garbage cans should be provided, collected garbage should be buried, sewage drains should be covered, sewage should be treated before disposal. 
• Control of vectors: Controlling growth and breeding of animals e.g. mosquitoes, flies etc. by spraying insecticides, eliminating the sites at which they breed. e.g. Stagnant pools.

Immune system of our body 

• Immunity is the ability of the body to resist infections. 
• The immune system of our body is normally fighting off microbes. We have cells that specialize in killing infecting microbes . 
• These cells go into action when microbes enter the body. 
• The immune cells manage to kill off the infection long before it assumes a major proportion. 
• Thus becoming exposed to or infected with an infectious microbe does not necessarily mean developing noticeable disease. 
• The functioning of the immune system will be good if proper and sufficient nourishment and food is available. 

Specific Methods

• These methods are disease specific. Specific prevention is provided by the immune system when it produces some specific molecules called antibodies to fight against specific invading microbes or their products called antigens. 
• Antibodies are proteins produced by white blood cells, called lymphocytes, in response to any foreign substances, called antigens. 
• Antibodies detect antigen, bind with them and destroy them. Antigens are usually proteins. They can be present on the surface of bacteria and viruses. 
• The harmful chemicals that bacteria and viruses secrete can also be regarded as antigens. When we suffer from a mild form of disease, our immune system forms antibodies against the invading antigens. 
• At the second infection of that disease, the immune system responds faster to destroy the antigens as there are antibodies that already recognize them. 
• As a result, we don't suffer from the severe form of the disease. 
• For example, if we had mild chickenpox once, we will not suffer from its severe form later. This forms the basis of immunization.

Immunization

• It is the process of stimulating the body to produce antibodies by artificial means. It can be done with the help of vaccines. 

Vaccines

• It is a preparation of weakened - infectious agents or their products that can be injected or given orally to prevent specific diseases. 
• This doesn't actually cause the disease but this would prevent any subsequent exposure to the infecting microbe from turning into actual disease. 
• Nowadays vaccines are available against tetanus, diphtheria, whooping cough, polio, chickenpox, measles, mumps, typhoid, hepatitis and many other diseases. 
• Many of these vaccines are given to children under the public health programme of childhood immunization.