The outbreak of Escherichia coli O157:H7 bacteria, better known as E. coli, that's affected romaine lettuce for two months has killed one person and sickened more than 120 people over 25 states.
Why is it taking so long to find the source of the outbreak and prevent more people from getting sick? It's because outbreaks connected to produce are particularly hard to trace.
It's now believed that the affected produce originated in Yuma, Arizona, where about 90 percent of the winter lettuce in the U.S. is grown. But pinpointing the farm or farms, the equipment that was used to harvest the crops or perhaps a water supply responsible for the contamination remains difficult. The Centers for Disease Control are continuing to investigate.
Romaine is processed multiple ways for consumption -- it's packaged, chopped or eaten whole. The contamination could be tied to one variety or multiple varieties. Again, it's very difficult to narrow it down.
Lettuce typically has such a short shelf life that by the time a person afflicted with E. coli can report it, tracing it back to a supply is challenging. What made that person sick probably isn't still at a supermarket or a warehouse where it could be collected and tested.
"Due to the 21-day shelf life we can't be certain that all of the product from this region is out of the supply chain," Peter Cassell, a Food and Drug Administrator spokesman, told ABC News.
Further complicating matters is that romaine lettuce often is mixed with other greens in prepacked salads, which could be contaminated at several stages of that processing. And lettuce is eaten by so many people in so many different ways, it's hard for those who get sick to say with certainty which meal from which day caused their illness.
The agricultural region around Yuma is huge, about 225,000 acres, with several farms and processing plants. There aren't formal regulations for how produce is processed, making it even harder to target a bad batch -- there are no batch numbers or bar codes.
What safety rules exist for events such as this outbreak? The Food Safety Modernization Act, passed in 2011, was designed as a preventative measure to reduce outbreaks from foodborne illnesses. Its goal is to standardize safety measures around the handling of produce and to monitor those protocols, because unlike outbreaks that come from beef, E.Coli 0157:H in foods that are eaten raw won't be killed by cooking them.
Unfortunately, required compliance with the regulations just began in January of this year and won't fully be in effect until mid-2019.
Chantel Strachan, MD, is a second-year medical resident from the University of Connecticut working with the ABC News Medical Unit.