This is a single celled parasitic animal, i.e., a protozoa, that infects predominantly humans and other primates. Most infections occur in the digestive tract but other tissues may be invaded. Onset time is highly variable. Theoretically, the ingestion of one viable cyst can cause an infection. Since cysts are not shed constantly, a minimum of 3 stools should be examined.
Serological tests exist for long-term infections.
It is important to distinguish the E. histolytica cyst from the cysts of nonpathogenic intestinal protozoa by its appearance. Amebiasis is transmitted by fecal contamination of drinking water and foods, but also by direct contact with dirty hands or objects as well as by sexual contact. It is also frequently diagnosed among homosexual men. In fewer cases, the parasite invades the soft tissues, most commonly the liver. Fatalities are infrequent.