Answer: Cryptosporidium spp.
C. parvum and D. hominis are the 2 main species causing human disease.
As some of the commentors mentioned, this case demonstrates the classic site and morphology of this parasite - small, round, 5-7 micrometer diameter parasites with an intracellular, extra-cytoplasmic location in the intestine. As Alasdair mentioned, cryptosporidiosis can be fatal in HIV+ and is one of the most common infections associated with HIV/AIDS.
Additional tests that could be performed for confirmation include stool examination with modified acid fast or safranin stains, or antigen detection methods. The latter is typically the most sensitive method for detecting Cryptosporidium.