Cryptosporidiosis, or "crypto" for short, is a disease caused by a group of organisms called Cryptosporidium. The organisms infect the lining of the small intestine and can cause severe diarrhea and malabsorption (an inability to absorb nutrients).

Cryptosporidium is spread by feces. Food or water contaminated with animal feces may carry these organisms; it's also possible that oral-anal sex may spread the infection. On occasion, there are outbreaks of cryptosporidiosis that can be traced to a feces-contaminated water supply. This is most common in warm-weather climates.

While anybody exposed to Cryptosporidium can get sick from it, diarrhea and malabsorption is usually limited to a few days in people with healthy immune systems. People with compromised immune systems—usually people with a CD4 cell count below 100—may experience prolonged and severe bouts of diarrhea and malabsorption that can be difficult to treat. In rare cases, the infection can cause pancreatitis or lung infections.