Unicellular fungi that live as parasites inside cells of invertebrates and vertebrates. Many of the 1200 known species cause disease (microsporidiosis) in a range of animal hosts, including humans (chiefly in immunocompromised subjects, such as people with HIV infection). Microsporidia lack cell walls and certain organelles typically found in eukaryotes; notably they have reduced structures called mitosomes instead of mitochondria. They produce resistant spores with chitin-containing walls and a coiled polar tube used to infiltrate host cells.