A mysterious outbreak of a stomach illness called cyclospora has proved particularly hard to trace.
The outbreak is growing, with more than 400 confirmed cases in 16 states. FDA officials said Friday that they had discovered the source of some of the illnesses, but not all of them.
Around half of the cases from Iowa and Nebraska are linked to salad mix from a Mexican farm that was served at Olive Garden and Red Lobster restaurants.
The rest of the illnesses are still a mystery.
Many doctors don't test for cyclospora because it is relatively rare. And the parasite is so tiny that it's often difficult to detect.
Even if they test, doctors may not notify state health departments, and states may not report the illnesses to federal health authorities.