Class 9th Science: Chapter 13 - Why Do We Fall Ill


Health and Disease
Health is a state of complete physical, mental and social well-being of an individual. Health is often affected by diseases. Disease is the unhealthy condition caused by microorganisms, improper diet or it is inherited. “Disease” literally means being uncomfortable. A person affected by the disease will have improper functioning of the organ systems.  Different symptoms which could be established during the diseased condition include headache, cough, loose motion and wounds with pus.

Classification of diseases
Diseases are categorised based on prevalence, occurrence or spread and duration.
•  Epidemics are the diseases based on prevalence that attack many people at the same time. e.g. plague.
•  Infectious diseases are the diseases based on occurrence or spread and duration.  These are the diseases caused by micro-organisms like bacteria, virus, fungi and protozoa. These are spread from one person to another through air, food, water etc.
•  Non-infectious diseases are the diseases based on occurrence or spread.  These are the diseases caused by genetic abnormalities.  They are called non-communicable diseases as they do not spread from one person to another. e.g. cancer.
•  Acute diseases are the diseases based on duration.  These diseases last for a short time without posing adverse effects on the health. e.g. common cold.
•  Chronic diseases are the diseases based on duration.  Chronic diseases last for prolonged period with drastic effects on health. e.g. Diabetes.

Diseases can also be categorised based on level of causes.  These can be immediate or contributory cause.
•  Immediate cause of the disease can be the microbes causing the disease. e.g. Bacteria.
•  Contributory causes include causes which contribute to immediate cause. e.g Genetic difference or poor nourishment.

Infectious agents
Organisms which cause disease are called as infectious agents.  They obtain nourishment from the host. e.g. Bacteria, Viruses, Fungi, Protozoa, Helminths.
•  Bacteria cause diseases like typhoid, cholera, tuberculosis, anthrax, pimples and peptic ulcers.
•  Viruses cause the common cold, influenza, dengue fever, SARS and AIDS.
•  Fungi cause skin infections.
•  Protozoa causes malaria, kala – azar and sleeping sickness.
•  Helminth worms cause intestinal infections or elephantiasis.

Target organs
These are the different parts of the body attacked by infectious agents. Mode of entry of microbes specifies the site of localisation. Symptoms tell us about the target organ infected.
•  If the symptoms are cough and breathlessness, then the lungs are the target.
•  Jaundice is a symptom of the liver, which is a target organ.
•  If the symptoms are headache, vomiting, fits or unconsciousness, then the brain is the target.

AIDS is a viral disease caused by a virus named Human Immuno Deficiency Virus or HIV.  HIV attacks the immune system of an individual and hence he cannot fight back many diseases. HIV finally causes death.

Transmission of Diseases

Transmission Transmission is the process of passing something from one individual to another individual. Infectious diseases are spread through microbial organisms from an infected person to a healthy person.

Microorganisms are transmitted through physical contact, air, water and carriers.
•  Air borne diseases:These are the diseases transmitted through air. Microorganisms are transmitted in the form of droplet nuclei in the air. e.g. common cold, pneumonia and tuberculosis.
•  Waterborne diseases: These are the diseases transmitted through water. Excreta of infected animals and humans contaminate drinking water. e.g. cholera
•  Physical contact: These are the diseases transmitted through direct contact. e.g. AIDS is transmitted from an infected mother to her child during pregnancy. AIDS is also transmitted through intimate contact between partners.
•  Carriers: The most common carriers or vectors that transfer micro-organisms from one human being to another are female mosquitoes, dogs, hens, etc. e.g.  Rabies virus transmitted by dogs and cats bite.

Effects of infectious diseases
Immune system in our body is activated. It functions to attack the foreign invaders and protect the body.
Inflammation is the response exhibited by the active immune system to kill the infectious agents. Inflammation includes swelling, pain, fever etc.

Treatment of infectious diseases
The two ways to treat infectious diseases are by reducing the effect of the disease and by eliminating the cause of the disease.
Antibiotics are the medicines used to treat the diseases caused by bacteria, fungi and protozoans. e.g. penicillin
Anti-viral drugs are the medicines used to treat diseases caused by viruses. These drugs are difficult to be prepared when compared to anti-bacterial drugs.

Prevention of infectious diseases
•  Airborne micro-organisms can be prevented by providing living conditions which are not over crowded.
•  Water-borne micro-organisms can be prevented by providing safe drinking water.
•  Vector-borne diseases can be prevented by providing clean environment.
•  Public hygiene is a basic step in the prevention of infectious diseases.
•  Proper and sufficient food is necessary for the immune system to work properly.
•  Immunisation through vaccine.

Immunisation and vaccine
Immunisation is the protection of individuals from communicable diseases by administration of a suspension of dead micro-organisms.

Vaccine is a biological preparation that improves immunity to a particular disease after inoculation. e.g. smallpox vaccine was the first vaccine developed against small pox by Edward Jenner, hence called "Father of Immunology".

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