Primary amebic meningoencephalitis: A rare, but fatal infection of the central nervous system caused by Naegleria fowleri, an ameba that inhabits freshwater lakes, rivers and hot springs. The disease results when ameba-contaminated water incidentally enters the nose during aquatic activities and amebas migrate to the brain. Symptoms can include headache, fever, stiff neck and seizures. Most cases occur in the summer and among children. High water temperature and lowered water level may lead to increased risk of infection. Recommendations for prevention include holding the nose shut or using nose clips when jumping or diving into bodies of fresh water.