Michigan has been the hardest hit state, and health officials said Friday that the number of infected has risen to more than 8,100.

Residents affected by the parasite told the BBC it was “the most painful thing I have ever been through in my life.”

In its update on Friday, the CDC warned people not to eat recalled Taylor Farms iceberg lettuce from central Mexico.

Taylor Farms, a major U.S. food supplier, voluntarily recalled all of its iceberg lettuce from central Mexico on July 17. The company said it did so “out of an abundance of caution.”

Investigators with the Food and Drug Administration announced over the weekend that iceberg lettuce from Taylor Farms had tested positive for cyclospora, but later retracted that statement, saying it was a false positive.

Health experts who spoke to the BBC said it is often difficult to trace the origins of cyclospora, the parasite that when consumed causes the intestinal disease cyclosporiasis.

Part of the challenge, experts said, is that symptoms may not appear until weeks after the parasite has been ingested, making it difficult for researchers to track what someone has eaten and find a common link.

The investigation into the multi-state outbreak linked to iceberg lettuce is expected to move slowly, Wade Syers, a food safety expert at Michigan State University, told the BBC.

“For Cyclospora, it is not unusual for the picture to clear up a couple of months after the first illnesses occur,” he said in an email Friday.

“Because, unfortunately, these investigations are more like marathon investigations than a sprint.”