Encephalitozoon cuniculi is a eukaryotic organism belonging to the phylum Microsporidia, in the kingdom Fungi. An obligate intracellular parasite, E. cuniculi occurs in laboratory mice and rabbits, monkeys, dogs, rats, birds, guinea pigs, and other mammals including humans. At various early times it was thought it was the cause of rabies and polio. This parasite may be transmitted in body exudates or transplacentally. Although damages are minimal, an infection can be fatal, especially in immunocompromised individuals such as AIDS patients.