Release of Cryptosporidium parvum sporozoites from oocyst, computer illustration. The parasite is transmitted to humans after ingestion of oocysts found on unwashed food, then it develops in the protrusions (microvilli) of epithelial cells that line the intestinal wall. Severe infection causes the folds of the intestinal wall to fuse and atrophy. Infection typically produces mild symptoms of diarrhoea, fever and headache. However, in the immuno- compromised, such as those with AIDS (acquired immune deficiency syndrome), infection can be fatal.