Cryptosporidium

Cryptosporidium is a protozoan parasite which causes outbreaks of diarrhea. 
A number of species of "Cryptosporidium" can infect mammals. The most common species to infect man is "Cryptosporidium parvum". The diarrhea can become severe in children, and in people whose immune system is weakened. It is often found in HIV patients who have diarrhea. In healthy adults, the infection is usually brief.
It is difficult to identify in the laboratory. The oocytes can be seen under a microscope, but they may be confused. Most cryptosporidia are 3-6 μm in size, but some reports have described larger cells. New real-time monitoring can detect "Cryptosporidium" better than the older spot testing and batch testing methods.
If "Cryptosporidium" is suspected in the water, the safest option is to boil water used for drinking.