See this question in NBME 27

Cryptosporidiosis presents with severe watery diarrhea in immunocompromised patients; it is diagnosed by microscopy of the stool showing red-pink oocysts on the acid-fast stain.

The patient with HIV infection and 10 days of watery diarrhea and abdominal cramps has been infected with Cryptosporidium, an intracellular parasite that causes a severe GI illness in an immunocompromised patient.