Primary amebic meningoencephalitis (PAM) is an acute, fulminant, and rapidly fatal infection involving the central nervous system. It is caused by the parasite Naegleria fowleri, a free-living ameboflagellate found in soil and fresh or brackish water (lakes, rivers, ponds). PAM develops with a rapid onset within several days of exposure and causes death within 4 to 5 days, often partly because of delayed diagnosis. Very few survivors have been reported (fewer than 12 of an estimated 200 cases).

The life cycle of N. fowleri has three stages: the trophozoite, a temporary flagellar stage known as ameboflagellate, and the cyst. The vegetative or feeding stage is the trophozoite stage of the ameba. This form is found in cerebrospinal fluid (CSF) or in tissue. In tissue, trophozoites ingest red and white blood cells, causing destruction. The ameboflagellate stage is a temporary form in which it neither feeds nor divides. This stage occurs when the trophozoites are exposed to a change in ionic concentration, such as in distilled water. It reverts back to the trophozoite stage within 24 hours. The resistant, protective form of the parasite is the cystic stage, which is never seen because the infection is so rapid and fatal that the patient typically dies before it occurs.