Any physical or functional change from the normal state that causes discomfort, or disability, or impairs the health of a living organism is called a disease. It occurs due to many reasons. A common cause of diseases is the lack of nutrients in our diet. It can also occur due to the presence of germs or microbes in our environment. There are two types of diseases such as communicable and non-communicable diseases.

A pathogen is an organism that causes disease in its host. Pathogens include viruses, bacteria, protozoans, and fungus. The pathogen then utilises the host body’s resources to replicate before exiting and spreading to a new host. Pathogens come in a variety of forms and can be transmitted by several routes, including skin contact, bodily fluids, and contact with contaminated surfaces. Some pathogens can be found as airborne particles.

The invasion of an organism's body by disease-causing agents are called infection. The germs may be bacteria, viruses, yeast, fungi, or other microorganisms. The diseases can be transmitted from one person to the other due to infection. The diseases that are transmitted due to infection are called an infectious disease or communicable disease. Examples: AIDS, Hepatitis B, etc.

An epidemic is an outbreak of disease which spreads rapidly from one place to another affecting a large population. Some examples are as follows: Yellow fever (The Aedes aegypti mosquito spreads a virus that causes the disease), smallpox (caused by the Variola virus), measles (caused by Morbillivirus), and polio (caused by Polio virus). Meningococcal infections (caused by the bacteria Neisseria meningitidis ) West Nile fever