Microsporidia are ubiquitous obligate intracellular parasites of all major animal lineages. They are important pathogens of mammals and invertebrates, and are now one of the most common infections amongst immuno-compromised humans. Microsporidia are unusual eukaryotes with very small genomes (as small as 2.3 MB), highly reduced biochemical pathways and pared down mitochondria called mitosomes.
Whilst the external spore is a biochemically dormant stage, host cell infection involves the rapid emergence from dormancy and the expulsion of a polar tube. This pierces the host cell membrane spore contents is directly transfer of the into the host cell cytoplasm. This intimate association between parasite and host appears to allow the microsporidian direct contact with host cell environment and to the resources available within it.