Isosporiasis is a disease caused by the protozoan Isospora belli. The organism infects the lining of the small intestine and can cause severe diarrhea and malabsorption (an inability to absorb nutrients).
Isospora belli is spread by feces. Food or water contaminated with animal feces may carry this organism; it's also possible that oral-anal sex may spread the infection. On occasion, there are outbreaks of isosporiasis that can be traced to a feces-contaminated water supply. Isosporiasis is quite rare in the United States; it is most common in tropical parts of the world and places where water contamination is a problem. In the U.S., isosporiasis is an initial AIDS-defining illness in approximately 0.2% of patients with AIDS.
People with compromised immune systems—usually people with a CD4 cell count below 150—may experience prolonged and severe bouts of diarrhea and malabsorption that can be difficult to treat. It's also important to note that not all people exposed to Isospora belli, even if their immune systems are suppressed, experience symptoms of the infection.