An infection caused by several species of the protozoan parasite microsporidia. Microsporidia usually infect the gastrointestinal tract but may also infect other parts of the body, such as the eyes, respiratory tract, or brain. The most common symptoms are chronic diarrhea and wasting, but symptoms vary greatly depending on the site of infection. Microsporidiosis occurs primarily in people with weakened immune systems, especially people with HIV or people who have undergone organ transplants.