The diseases which spread from one person to another by contact are known as contagious diseases. The main causes of infection are contact with infected people or by eating infected food or water. The infections are caused by micro-organisms, also germs, which can be seen only under the microscope. There are several types of micro-organisms such as bacteria, virus and protozoan.

Bacterial diseases
Bacteria are micro-organisms that live in human food. Some of them are harmful. For example, food poisoning is caused by harmful bacteria like moulds in bread. Bacteria can also cause many communicable diseases like typhoid, pneumonia, etc.

Virus is a group of organisms that cause common cold, polio and measles.
Protozoan is small microscopic animals that cause diseases like malaria, dengue, etc.

Spread of diseases

·    Through air
The common cold, tuberculosis and whooping cough spread from person to person through air, when an infected person coughs the germs spread in the air. A healthy person can be infected easily by breathing this air. Always sneeze or cough into a handkerchief.
·    Through water 
Typhoid, cholera and jaundice are spread by infected water. To kill these organisms, filter and boil water before drinking.
·    Through contact
Some diseases spread by direct contact or by contact with the towels, bed sheets, combs, cups and food of a person infected with the disease. Wash all times before using them. Clean the room of the patient with disinfectant.
·    Through a vector
Some animals and insects carry the disease-causing micro-organisms. Dogs spread rabies and mosquitoes spread malaria. The disease is spread directly when the dog or mosquito bites a person. The dog or mosquito becomes a vector.

The mosquito sucks blood from a person infected with malaria. The malarial parasite lives in the salivary glands of the mosquito. When the mosquito bites another person, it passes on the infection.

The plague bacterium is transmitted to man mainly through rat bite. The rat flea acts as a vector in spreading the disease from an infected rat.

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