Cryptosporidium parvum parasites, the cause of cryptosporidiosis, computer illustration. The parasite 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.