The outbreak, June 26 -- For most of the people now counted among this summer's Cyclospora cases, the illness began unremarkably: a cramping stomach, a loose stool, an afternoon spent near a bathroom that felt, at first, like a minor inconvenience. Then it lasted two weeks. Then three. The fatigue set in, and the weight loss, and the frustrating conviction that something was wrong that no one could quite name. Cyclospora cayetanensis, the microscopic parasite responsible for 145 confirmed cases across 17 U.S. states as of June 16, is the kind of illness that defeats diagnosis precisely because it mimics something ordinary, right up until the moment it does not.

Cyclospora infection is not included in standard stool panels at most hospita...