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Cryptosporidium
Cryptosporidium are a genus of single-cell parasites that often infect calves and occasionally humans, but also up to 40 more vertebrates. They belong to the Apicomplexa and are closely related to Plasmodium, the pathogen of malaria, and with Toxoplasma gondii, the pathogen of the toxoplasmosis.
Kryptosporitien form two types of oocysts. About 80% are walled and are eliminated with feces. The remaining 20% are thin wall and remain in the host where they trigger a re-infection of the host.
The thick-walled oocysts are very resistant and can remain infectious for several months under favorable conditions (humidity and temperature). They are insensitive to many disinfectants.
(Sources: Dögens J. (1988): Parasitology; Thieme Stuttgart; H. Mehlhorn and G. Piekarski: Plan of the parasite-customer, Heidelberh, 6th edition. 2002; Kayser: Medical Mikrobioligie, 12 ed. 2010, Thieme Verlag)