Infection

From Citizendium, the Citizens' Compendium

Infection happens when one organism invades (infects) the body of another, unwitting and usually much larger organism, and this results in harm to the "host" organism. Organisms that can cause infection include bacteria, fungi, viruses and parasites. The type and severity of infections range greatly, from a mere annoyance or embarrassing condition, such as a fungus under the nail, which one can live with for years, to a severe condition or life-threatening disease. Some infections are of short duration and go away of their own accord or with simple remedies, such as the common cold or thrush on the tongue or vaginal yeast infections.

Human beings often speak of being "attacked" by an invading organism, because that is how we perceive the infection, especially when it causes us illness or pain. In reality, such organisms are simply seeking hosts on which to feed or in which to complete their own life cycles.