The human intestinal fluke (Fasciolopsis buskii) is the source of the HIV virus. This parasite typically lives in the intestine where it might do little harm, causing only colitis, Crohn's disease, or irritable bowel syndrome, or perhaps nothing at all. But if it invades a different organ, like the uterus or the kidneys or liver, it does a great deal of harm. If it establishes itself in the thymus, it causes HIV/AIDS!