The US Centers for Disease Control and Prevention posted an update Friday on the Escherichia coli (E. coli) outbreak associated with romaine lettuce from the Yuma Valley growing region, reporting four more deaths for a grand total of five since the beginning of May. But, like, why? The CDC first announced the outbreak way back on April 10. So why are we still hearing about it?

The simple answer is that outbreak investigations like this one take time. When someone gets sick with E. coli from eating contaminated romaine lettuce, there’s not some super-high-tech detector that pings their location to the CDC and marks it on a map. Investigators from the CDC and the Food and Drug Administration have to look at hospital records, conduct interviews with patients, and inspect food production facilities before they can be sure whether someone’s sickness was actually caused by a particular food from a particular place. At this point, all of the affected lettuce is way past its expiration date, but we’re still hearing about cases because that’s just how these things go.

“It takes two to three weeks between when a person becomes ill with E. coli and when the illness is reported to CDC,” according to the CDC’s latest announcement. “The most recent illnesses reported to CDC started when romaine lettuce from the Yuma growing region was likely still available in stores, restaurants, and in peoples’ homes.” The announcement, posted on Friday, indicates that 23 more illnesses across 13 states had been confirmed since the last report on May 16.

Just to be clear, this doesn’t mean that 23 more people got sick since May 9, or that infected romaine lettuce just arrived in Arkansas, North Carolina, and Oklahoma — the three latest states reporting E. coli cases. All it means is that the CDC has only now confirmed that their E. coli was most likely caused by eating contaminated romaine lettuce.

The CDC reports that romaine lettuce has a shelf-life of about 21 days, and the last infected shipment from the farm the outbreak was traced back to left on April 16, so quick maths tell us that even the most recent crop of E. coli lettuce was wilted and spoiled by May 7.

The CDC’s Friday report was not labeled a “final report” on the outbreak, so it’s possible that a few more cases will continue to trickle in over the coming days and weeks. But at this point, it’s safe to say that you can go back to scarfing down Caesar salads, wrapping bunless hamburgers, and whatever the heck else you use romaine lettuce for.