Here's a nice post on Boing Boing from Maggie Koerth-Baker explaining the challenges involved in identifying the source of an E. coli outbreak:
. . .outbreaks of food-borne illness, in general, are tricky to pin down. Just think about the way you eat—lots of different foods, from different stores, coming from different places ... even different countries. You don't know where it came from. And you probably don't even have the best recall of what you specifically did and didn't eat over the course of the past week. That's important, because investigations of food-borne illness start with interviews. Epidemiologists compare the foods that sick people reported eating and compare them to the ones that healthy people ate. Any dish that shows up more frequently among the sick becomes a suspect. But then scientists still have to narrow down the ingredients. It's a slow investigation, and an imperfect one that, by its nature, can only happen long after the trails of evidence start to become fuzzy. . . .
There are also some very nice images of the process in her entry.
And, if you would like to read a little further, there's also a good entry on the Nature News Blog that points to some of the same challenges and a potential wrinkle:
Questions are also being raised about why Shiga-toxin-producing bacteria have become far more common over the past few decades – with some scientists suggesting to Nature that agricultural use of antibiotics may be a factor.
Previously: The Big Picture captures the E. coli outbreak in Europe
Photo by Little Blue Hen