Clin Infect Dis ciab549, 2021. doi: 10.1093 / cid / ciab549 Parasitic gastroenteritis The parasites most commonly implicated in high-resource countries are Giardia Cryptosporidium Certain intestinal parasites, notably Giardia intestinalis (G lamblia), adhere to the intestinal mucosa, causing nausea, vomiting, diarrhea, and general malaise.