Cryptosporidium parvum causes watery diarrhea. Disease is often severe in immunocompromised individuals (for example, people with AIDS or patients receiving cancer therapy).
Infection is acquired by ingesting oocysts.
It typically presents with:

Cryptosporidium parvum oocysts stained with modified acid-fast. Against a blue-green background, the oocysts stand out in a bright red stain. Sporozoites are visible inside the two oocysts to the right in this image.