The intestinal parasite Cryptosporidium, which causes a diarrheal disease, is very good at infecting humans. It’s the leading cause of waterborne disease from recreational waters in the United States. Globally, it’s a serious illness that can stunt the growth of, or even kill, infants and young children. And people with compromised immune systems, such as those with HIV/AIDS, are also highly susceptible. There is no vaccine and no effective treatment.