Microsporidiosis is a parasitic infection of epithelial lining cells with microsporidia species. Traditionally, microsporidia were classified as protozoans but are now considered very closely related to fungi. Of 14 species that can infect humans, Enterocytozoon bieneusi and Encephalitozoon intestinalis are the most common microsporidia. The target organs of infection and the clinical presentations depend on the infecting species. The status of a patient's immune system will further dictate the severity and spectrum of infection. Microsporidia are a cause of chronic diarrhea in patients with human immunodeficiency virus (HIV) infection. Microsporidiosis has also been reported in patients who have undergone solid organ transplant (eg, renal, liver, heart).

The sites of E. bieneusi infection are the epithelial cells of the duodenum, jejunum, and bile ducts. Clinical manifestations of infection with this species include self-limited diarrhea, chronic diarrhea, malabsorption, cholangitis, and cholecystitis. Enterocytozoon bieneusi infection is one of several infectious etiologies in traveler's diarrhea. The infection can be found worldwide.

• In healthy hosts, including elderly hosts, the usual presentation is self-limited diarrhea.
• Asymptomatic infection has been reported in children.
• For immunocompromised hosts, the common symptoms from the infection are chronic diarrhea and malabsorption. Both cholangitis and cholecystitis have also been reported in this group of patients when infected with E. bieneusi. HIV-infected patients with CD4+ T cell count less than 100 per microliter are still commonly infected with microsporidia in the developing world. Antiretroviral therapy has significantly decreased cases of microsporidiosis.
• However, the infection has been diagnosed increasingly in other immunocompromised patients who require immunosuppressive medication for organ transplant, or treatment of cancer or autoimmune diseases. Disease presentation and course in transplant patients are similar to that seen in HIV-infected patients, with persistent diarrhea, vomiting, and weight loss.   

Encephalitozoon intestinalis is the second most common microsporidiosis. The organism can directly infect the small intestine, respiratory tract, urethra, prostate, or eyes.

• In immunocompetent hosts, the disease may present as a self-limited diarrhea, eg, in travelers, or a keratoconjunctivitis in contact lens wearers or after corneal trauma.
• The infection is more frequent in immunocompromised hosts. In these hosts, especially patients with AIDS, infection with Encephalitozoon species can disseminate from the site of entry to other organs, including the muscle, brain, kidney, and liver. Clinical presentations vary from chronic diarrhea, malabsorption, cholangitis, and cholecystitis to sinusitis, tracheobronchitis, keratoconjunctivitis, myositis, encephalitis, interstitial nephritis, and hepatitis in patients with AIDS.

There have been more recent reports of infection with other microsporidia species outside the small intestine in immunocompromised hosts. For example, Brachiola algerae, one of the 14 species, caused fatal myositis in a woman receiving methotrexate for treatment of rheumatoid arthritis.
    
Microsporidia can be found in the environment. Potential reservoirs are infected humans, animals, and contaminated water. Possible routes of infection are fecal-oral, oral-oral, inhalation of contaminated aerosols, direct contact to the eyes, and ingestion of contaminated food or water.