Infectious diseases are caused by pathogens and can be transmitted from one person to another.

Disease are disorders that affect the structure and function causing unpleasant symptoms.

Cholera is caused by bacteria Vibrio cholera

It is transmitted through contaminated water and food. The bacteria enter the body and divide in the intestine where it secretes toxin which inhibits reabsorption of water and salts in the intestine. These are lost in feces causing diarrhea. The bacteria is excreted in the feces of the affected individual. 

It can be prevented by treating sewage water properly and promoting good hygiene.

Malaria is caused by plasmodium. There are different species of plasmodium which causes malaria. It infects the RBCs where it multiplies. After some time the RBCs are burst and plasmodium is released into the blood causing fever. 

Malaria is transmitted by anopheles mosquito which serves as a vector. Plasmodium completes its sexual life cycle in mosquito producing gametes which are released into the blood of the individual when the mosquito bites him. It then completes its asexual life cycle in man and produces sporozoites which are taken up the mosquito.

Malaria can be prevented by reducing the population of mosquitos or by preventing bites by using mosquito repellants.

Tuberculosis is caused by bacteria Mycobacterium Tuberculosis. The primary infection occurs in the lungs. From the lungs, the bacteria can break into the blood and cause secondary infection in other sites such as bones or intestine.

Tuberculosis is transmitted through aerosols of an infected individual. The infection usually occurs in crowded areas.

The vaccine is available for tuberculosis called BCG which is given at the time of birth. The infection is usually prevalent in people who are malnourished or HIV infected.

AID is caused by a virus called the human immunodeficiency virus. It is a retrovirus that contains RNA. The virus infects T lymphocytes and uses the cell DNA to make a copy of itself. The virus remains dormant in the cell for many years but it can activate in the body anytime and make a copy of itself. During this process, it destroys the T lymphocytes due to which the number is decreased so much that the individual is unable to fight infections.

It is transmitted from the exchange of body fluids. This can happen via:

- Use of unsterile needles.

- Blood transfusion.

- Unprotected sexual activity.

- Mother to the fetus via the placenta.

Hiv can be prevented by using sterile needles, blood screening, using condoms during sexual intercourse.

Mother to fetus transmission is reduced through drugs use to treat HIV.

Measles is caused by morbillivirus.

Measles is transmitted through the airborne droplet. It is highly infectious and can be transmitted to a large number of population.

The disease can be prevented by using vaccines. It usually affects those who are malnourished.

Smallpox is caused by the variola virus

It is transmitted through airborne droplets.

Smallpox has been completely eradicated by WHO through vaccination campaigns.

Malaria is common in countries where anopheles mosquito is present. This includes a tropical and subtropical region where humidity is high.

TB is prevalent in areas where living conditions are poor or in countries where HIV is present.