Naegleria fowleri is the agent of a severe purulent meningoencephalitis: the "Primary amoebic meningoencephalitis". N.fowleri are free living amoebas that live in warm fresh water all over the world. 
The lyfe cycle consist of three stages: the amoeboid growing form that lives in the mud and at the bottom of the ponds; the rapidly motile biflagellate trophozoite who lives in surface layers of water (the infective form); the dormant cyst. 
Primary amoebic meningoencephalitis occurs in individuals who have been exposed to freshwater lakes or ponds usually during swimming; the incubation period is about 3 to 7 days (but it may last up to 2 weeks). Invasion of the CNS occurs after nasal inhalation of contaminated water containig the biflagellate trophozoites; trophozoites (in the amoeboid form once arrived in the nasal cavity) penetrate the epithelium and enter the CNS through the olphactory nerve branches in the cribriform plate and cause a purulent meningoencephalitis. 
Trophozoites are 10-to 30 m m in diameter and have a clear nucleus with a prominent dense central nucleolus; the cytoplasm contains mytochondria and the rough endoplasmic reticulum; usually ingested red blood cells, leukocytes and bacteria are visible. 
Cysts are 9 m m in diameter; they are spherical with a central nucleus.