(CNN) - Don't worry, the recent deadly outbreak of E. coli in romaine lettuce is over. And yes, the romaine lettuce you buy at the store or pile on your plate at the salad bar now is safe to eat.
But just how did that lettuce become contaminated in the first place? The answer might be in the water.
According to an outbreak update from from the U.S. Centers for Disease Control and Prevention issued Thursday, romaine lettuce contaminated with a particularly virulent strain of E. coli O157:H7 sickened 210 people in 36 states between March 13 and June 6. The age of victims ranged from 1 to 88 years of age.
Ninety-six of those folks were sick enough to be hospitalized; 27 of those developed hemolytic uremic syndrome, a type of kidney failure. Five people from four states died. It's the worst outbreak of E. coli 0157:H7 since a 2006 outbreak linked to spinach.
Using a special type of DNA fingerprinting called whole genome sequencing, the CDC was able to determine that the E. coli bacteria sampled from those who became ill was closely related genetically, so a single source of infection was the most likely cause.
The U.S. Food and Drug Administration, along with the CDC and various state partners, then traced the outbreak back to a single growing region: Yuma, Arizona, which calls itself the "Winter Lettuce Capital of the World."
It's not only lettuce that Yuma supplies. Thanks to rich soil created from Colorado River sediment and some great irrigation, Yuma grows about $2.5 billion a year of more than 175 different crops, including dates, lemons and melons. But what's important here is that Yuma County says it grows 90% of all leafy greens America eats between the months of November and March.
According to the FDA the last shipments of lettuce for the season shipped in April and the shelf-life has since expired therefore the contaminated lettuce is no longer available.
During the investigation, the FDA discovered that the outbreak couldn't be traced back to a single grower, harvester, processor or distributor. It was across multiple supply chains. That led to suspicions that the outbreak might be from a common water source.
On Thursday, the CDC said that indeed, samples taken from canal water that irrigated the Yuma growing fields were laced with the same deadly bacteria.
"The E. coli O157:H7 found in the canal water is closely related genetically to the E. coli O157:H7 from ill people," the CDC said in a final update on the outbreak.
How the E. coli came to be in the canal water is still under investigation by the FDA.
"Samples have been collected from environmental sources in the region, including water, soil, and cow manure. Evaluation of these samples is ongoing," the FDA said in an update.
Interestingly, not all of the people who became sick had actually eaten romaine lettuce. Some had close contact with people who had eaten the infected greens. The CDC reminds everyone to always use safe handling practices with any fruit or vegetable.
"Important steps to take are to cook meat thoroughly, and wash hands after using the restroom or changing diapers, before and after preparing or eating food, and after contact with animals," advises the CDC.
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