Amoebic parasitoma

An amoebic parasitoma is a cyst formed by several species of ameobic and protozoan parasites in order to protect themselves from the immune system or other harsh environments within a host, such as the stomach. Many parasites form cysts as part of their life cycle.
In most cases, such cysts are small and asymptomatic, although they may allow the spread of the parasite to another host. However, in rare cases, a cyst can grow large - isolating itself from the rest of the body and causing no symptoms. When such a cyst is damaged, the parasites are often released into the bloodstream causing an overwhelming immune response, typical of such diseases as sepsis.
In most cases, treating the parasite will alleviate the symptoms of the patient.
Microbial cyst at Wikipedia