The FDA is investigating another outbreak of the parasite Cyclospora

(Bloomberg) — The U.S. Food and Drug Administration said it is investigating another outbreak of the diarrhea-causing cyclospora parasite, with 72 cases linked to an unknown source.

The latest infections released by the agency on Wednesday add to a surge in cases concentrated in the Midwest. These were linked to Taylor Farms lettuce and the company recalled the products. The FDA has already investigated several other ongoing cyclosporine clusters.

Outbreaks worry consumers and scare investors. While the FDA said the latest outbreak was not linked to an identifiable product, shares of Sweetgreen Inc. were down more than 13% at 3:25 p.m. in New York, and Chipotle Mexican Grill Inc. fell more than 4% to a session low. None of the companies have yet been linked to the outbreaks.

Yum! Brands Inc., whose Taco Bell chain was previously linked to the Taylor Farms cases, fell as much as 1.4%.

Sweetgreen and Yum pared those losses by the end of trading, with Yum closing up slightly for the day.

The parasite is notoriously difficult to trace to a specific product or source. In humans, symptoms usually do not appear until two weeks after ingesting the parasite, long after the receipts have been discarded and potential clues to the cause of the disease have been forgotten.

The FDA’s addition of a new outbreak leaves a lot to the imagination — there are no other details about which states have been affected.

Cyclospora cases increase each summer as warmer weather allows the parasites to thrive. Thousands of people in 41 states have been infected so far this summer, according to the US Centers for Disease Control and Prevention. At least 300 have been hospitalized, the public health office said Tuesday.

There are usually fewer than 100 cases in the entire country in an entire year.

The FDA previously linked a false-positive lab test to Taylor Farms lettuce — and stopped short of expanding the existing recall. However, this move did not absolve the production company from the blame. The agency’s investigation with the CDC and state health departments found that most of the sick people ate at Taco Bell locations in the Midwest that came from Taylor Farms iceberg lettuce. The company said it is “voluntarily removing all iceberg lettuce originating in Central Mexico.”

The news and delays from Taylor Farms and the FDA have caused confusion among consumers about what is safe to consume.

(Update with more details throughout.)

More such stories are available at bloomberg.com