Cryptosporidium protozoa. Light micrograph of Cryptosporidium parvum protozoa (pink) in a human stool sample. If ingested, each oocyst releases infective sporozoites that attach to the surface of the intestines. These parasites cause severe diarrhea in humans, a condition known as cryptosporidiosis. This can be spread from person to person or from domestic animals to people. Diarrhea and abdominal pain typically last for around a week. The infection may be fatal in AIDS (acquired immune deficiency syndrome) sufferers, and other people with a suppressed immune system. Brightfield illumination. Magnification: x250 at 35 mm size.