Why we don’t know what food is spreading the parasite sickening thousand

There’s a lag between when people consume the parasite and when symptoms appear, making it tough for those infected to remember what they ate to pinpoint the problem.

More than 2,000 Americans have been sickened this summer by a microscopic parasite that contaminates fresh produce and can cause days of diarrhea, creating an unusually large outbreak that, paradoxically, may give investigators their best chance to identify its source, public health officials said.

Cyclospora is one of the hardest foodborne pathogens to trace to its source. There’s a lag between when people consume the parasite that causes the illness and when symptoms appear, making it tough for those infected to remember what they ate to pinpoint the problem. Health officials are alarmed by the rapidly growing number of cases, which they say are likely undercounted because some people recover without medical care and are not tested.

Authorities have not yet identified a specific produce grower, supplier, or type of produce responsible for the latest outbreak. But this season’s unusually high number of illnesses, now reported in at least 21 states, means more information and more patients to help identify.