The free-living amoebae that cause human infections include Acanthamoeba, Naegleria, Balamuthia mandrillaris, and, rarely, Sappinia. All 4 genera cause serious CNS or ocular infections. Distinct from enteric pathogenic protozoa, they all are usually soil/water commensals, have no human carrier state, involve no insect vector, and cause sporadic disease associated with specific behaviors and exposures.

Acanthamoeba are among the most prevalent environmental protozoa and have been classified by 18s rDNA sequencing into at least 20 genotypes, designated T1-T20. The most common environmental and human pathogens belong to the T4 genotype