Public-health nurses in Washtenaw County, Michigan, are interviewing people with cyclosporiasis and checking grocery receipts, banking records and food-delivery apps to identify a common source of infection. The work illustrates the difficulty of tracing foodborne outbreaks after the relevant meals have been eaten weeks earlier, and highlights concern that strained local and federal public-health systems may be less able to detect and contain such outbreaks.

Cyclospora is a microscopic parasite that can contaminate food or water and cause prolonged diarrhoea, abdominal pain, nausea and fatigue. The Michigan outbreak has affected thousands of people across dozens of states, while Washtenaw County staff have been diverted from work including immunisations and sexual-health services after also responding to a measles outbreak involving seven people, five of them children.