Naegleria is a microscopic ameba (single-celled living organism) that can cause a very rare, but severe, infection of the brain. The ameba is commonly found in warm freshwater (for example, lakes, rivers, and hot springs) and soil. Only one species (type) of Naegleria infects people: Naegleria fowleri.

Naegleria fowleri infects people by entering the body through the nose. This typically occurs when people go swimming or diving in warm freshwater places, like lakes and rivers. In very rare instances, Naegleria infections may also occur when contaminated water from other sources (such as inadequately chlorinated swimming pool water or heated tap water <47°C) enters the nose. Once the ameba enters the brain, it causes a usually fatal infection called primary amebic meningoencephalitis (PAM).

Image: L & R: Trophozoites of N. fowleri in brain tissue, stained with H&E (L) and trichrome (R). Center: Ameboflagellate trophozoite of N. fowleri. Credit: DPDx and GS Visvesvara.