More than 20 species of Acanthamoeba have been identified and are classified into three morphological groups. Group I has large cysts (≥ 22.6 µm) which have a rounded outer wall that is clearly separated from the inner wall. The cysts in Group II are smaller in size (≤ 18 µm) with an inner wall that is variable in shape. The cysts in Group III are smaller (< 18 µm) than Group II and have poorly separated walls. With one exception, the principle human pathogens belong to Group II. It was found, however, that the identification of Acanthamoeba species by only morphological features became problematic, since there are variations in cyst morphology belonging to the same strain. Byers in 1991 classified the genus based on RNA sequences. The genus has now been divided into 17 different genotypes (T1 to T17). The majority of human cases are associated with genotype T4. To date, no disease association has been found for genotypes T7 to T9 or T13 to T17. The ability of Acanthamoeba species to survive in diverse environments demonstrates the ubiquitous nature of this organism. Some of the areas where the amoeba has been isolated include soil, air, sea water, rivers, ponds, public water supplies, air conditioning units, sewage, contact lenses and their cases, bottled water, and vegetables.