[...] multiple visits to the doctor after a month with debilitating nausea and diarrhea didn't yield any answers. A mysterious outbreak of the parasitic illness usually found abroad is growing, with more than 400 confirmed cases in 16 states. The agency said that the illnesses from Iowa and Nebraska are linked to salad mix from a Mexican farm that was served at Olive Garden and Red Lobster restaurants. Doctors or labs may not notify state health departments as quickly as they would for a more common foodborne illness like salmonella. CDC spokeswoman María-Belén Moran says the agency also is interviewing people who aren't sick as controls to get more information on eating patterns, as well as lab testing foods that they suspect. For its part, the FDA says it has a 21-person team in its Maryland headquarters and specialists in 10 field offices across the country working to identify the source of the outbreak. Food often goes through several stops — potentially in several countries — before it reaches a grocery cart, and trying to race it is "labor-intensive and painstaking work, requiring the collection, review and analysis of hundreds and at times thousands of invoices and shipping documents," the FDA said.