Enlarge / A Wendy's old-fashion burger. Romaine lettuce on Wendy's burgers is thought to be the cause of the outbreak. (credit: Getty | Francis Dean)

We will never know for certain what caused a large, multistate outbreak of E. coli O157:H7 infections linked to Wendy's restaurants late last year, according to a new study led by investigators at the Centers for Disease Control and Prevention.

The study, highlighting weaknesses in our ability to respond to foodborne outbreaks, lands amid a separate report published by the CDC finding that, in general, we're also failing to prevent outbreaks. In fact, cases from some common foodborne pathogens have increased relative to pre-pandemic levels.

In the outbreak last year, which spanned from July to August, at least 109 people in six states fell ill, with 52 needing to be hospitalized. Eating at Wendy's was a clear link. But it wasn't enough to crack the case.