primary amebic meningoencephalitis (PAM), a rare and often fatal acute, febrile, purulent meningoencephalitis caused by free-living soil and water amebas of the genera Naegleria and Acanthamoeba. Infection caused by Naegleria is generally seen in young persons who swim or bathe in contaminated freshwater, the pathogens gaining access to the central nervous system by penetrating the nasal mucosa and cribriform plate and then following the olfactory bulbs and nerves to the brain and meninges. By contrast, Acanthamoeba infections tend to be more benign, are more often seen in older or immunocompromised persons, and are sometimes associated with spontaneous recovery; the mode of transmission of these infections is not known, but hematogenous spread from amebic infection at distant sites has been reported.
