Cystoisosporiasis (formerly isosporiasis) is caused by Cystoisospora belli (erstwhile named Isospora belli) is encountered globally, particularly in tropical and subtropical regions. Cystoisosporiasis is a human intestinal disease whose etiology is the parasite Cystoisospora belli with infection frequent in immunocompromised subjects, principally HIV-infected and AIDS patients. This coccidium parasite infects the epithelial cells and lining of the villi of the small and large intestines. C. belli is the least frequent of the three intestinal coccidia, viz: Cryptosporidium, microsporidium and C. belli which perturb humans. The clinical presentation of cystoisosporiasis gives a semblance of inflammatory bowel disease and irritable bowel syndrome, as well as other gastrointestinal symptoms, nausea, vomiting and diarhoea found in COVID-19, AIDS and HIV-infected patients. Research has not presented comorbid features of COVID-19 and cystoisosporiasis. The oocytes of C. belli are visualizable microscopically on wet mounts via bright-field, differential interference contrast (DIC) and epifluorescence. Trimethoprin sulfamethoxazole constitute the normal treatment of choice. C. belli,HIV-infected/AIDS and COVID-19 patients have clinicopathological correlates necessary to elucidate comorbidities and mechanisms of the diseases.