A drug used to treat the parasite behind a growing number of “explosive” diarrhea outbreaks comes with an ironic warning.

Health officials are racing to find the source of a parasite that has sickened potentially thousands of people across more than 30 states and isn’t showing signs of stopping.

"The most sensitive tests, meaning the tests that are most likely to detect an infection, are stool PCR tests, or you can use stool microscopy," Dr. Jeanne Marrazzo, CEO of the Infectious Diseases Soc...