Isosporiasis (also known as cystoisosporiasis) is an exclusively human parasitosis occurring mainly in the tropics and subtropics, due to infection with <i>Isospora belli</i> (through ingestion of contaminated food), that is frequently asymptomatic or that can cause fever and diarrhea, but that is usually a self-limiting condition in the immunocompetent. HIV-positive individuals are particularly at risk of suffering from symptomatic isosporiasis and can manifest with a more severe clinical course of chronic diarrhea and severe weight loss.

An infection that is caused by a pathogen that would generally not be able to cause an infection in a host with a normal immune system. Such pathogens take advantage of the opportunity, so to speak, that is provided by a weakened immune system.