Experts say what makes these types of investigations difficult is that it only takes a minuscule amount of E. coli contamination to trigger an outbreak. Finding the source for such a small amount of contamination is tricky at best.
What makes it even more difficult for consumers is there's little a person can do to clean lettuce or spinach that has been contaminated with E. coli. The FDA says it's not enough to wash or even cook vegetables if they've been contaminated. The only effective preventive measure is to discard produce that fits the established profile.
Dr. Robert Schooley, a researcher of infectious diseases and a professor at the University of California at San Diego, says this time federal health officials may not ever be able to identify the source of the E. coli contamination.
"The cases of infection are so widespread across the country, and E. coli is so ubiquitous in the environment, I wouldn't be surprised if they don't ever figure it out."