It is responsible for the condition isosporiasis, which causes acute, non-bloody diarrhoea in immunocompromised individuals.
At least 248 species have been described in this genus. For instance, the house sparrow has 12 species of Isospora. However, most species are little studied, and some authors doubted whether all should be recognized as distinct species. Some have proposed splitting those species with paratenic hosts into separate genera Levinia or Cystoisospora, but as of 1997 this had not been widely accepted.