Infectious disease

Infectious diseases are caused by pathogenic organisms, such as bacteria, viruses, parasites, and fungi. Infectious diseases can be passed from one person to another, either directly or indirectly, for example by touching contaminated surfaces or consuming contaminated food or water. They are distinct from other types of diseases like genetic disease (caused by mutations in the DNA), deficiency disease (caused by a deficiency, usually of vitamins or minerals) and physiological diseases (where an organ malfunctions), which are not contagious.

Infection is caused by an infectious agent entering the body and multiplying. This is called colonization. Most organisms, including humans, have large numbers of microorganisms living in and on their bodies. These microorganisms do not usually cause disease and can be beneficial or even necessary for the health of the host. Such microorganisms are called commensals, or the normal microbiota. Other microorganisms, however, can cause disease when the colonize the host. Commensal microorganisms may also cause disease when they colonize a part of the body outside their usual niche or when the host immunity is low.

In general, infectious disease is characterized by fever, redness or swelling at the site of the infection and pain. The severity and exact symptoms of the disease depends on multiple factors, including the virulence of the pathogen, the site of the infection, and the host defenses.