The American Heritage® Dictionary of the English Language, 4th Edition
n. A protozoan of the genus Cryptosporidium that is an intestinal parasite in humans and other vertebrates and sometimes causes diarrhea that is especially severe in immunocompromised individuals.
n. A protozoan, of the genus Cryptosporidium, that is an intestinal parasite of humans and other vertebrates; can cause diarrhea and other symptoms in cases of low immunity.
Word Usage
"So is an intestinal parasite called cryptosporidium, which can be fatal in some cases."