Those parasites which reach their full pathogenicity only in immunocompromied hosts (e.g., AIDS patients, cortisone drug users), while the same parasites lead to no or only to mild clinical symptoms in immunocompetent hosts were considered as opportunists. However, with respect to life cycle only those parasites which have an endogenous reproduction phase in humans, which may give rise to uncontrolled endoautoinvasions, may become opportunists. The most important opportunists in AIDS patients are Pneumocystis carinii, Cryptosporidium parvum, and Toxoplasma gondii , which are the main “killers” in American and European AIDS patients comprising of up to 70 % of the cases. Other spp., like the following are also found (often together and in large numbers) in such patients: the protozoans Giardia lamblia , Blastocystis hominis , Naegleria spp., Acanthamoeba spp., Entamoeba histolytica , Isospora belli , Cyclospora cayetanensis , many microsporidian species ( Microsporidia), Balantidium coli , the nematode Strongyloides stercoralis , and the mange mites Sarcoptes scabei and Demodex folliculorum . For more details see Opportunistic Agents, Man, Encephalitozoonosis.