It pains me to say this, because I'm lazy. But after the umpteenth outbreak of food poisoning from bags of freshly washed greenery, I think it's time we all stop eating bagged salads.
Photo from VisualHunt.
According to a 2015 US government report, fruit and vegetables are the #1 source of Listeria and Salmonella food poisoning, and a close #2 (sorry) for E. coli. But there's something about bagged lettuce that makes it worse than your average vegetable. Emily Bazelon explains, writing at Slate in the aftermath of an earlier E. coli outbreak:
To produce the bags, processing plants take greens from different farms, put them through three different chlorinated baths, dry and seal them in plastic, and then ship them to a market near you. The chlorination doesn't get rid of E. coli: To do that, you need to heat the leaves and treat them with an organic acid, which would probably make them go limp. So, by mixing greens from different farms without treating them for contamination, the processing of bagged spinach spreads E. coli once it's present in a particular field.
This is why I'm switching back to preparing my own salads. There's nothing you can really do if a bag of salad comes in with bacteria on it: washing it will just spread the germs around but still leave most of them alive. (If you want to wash it anyway, a vinegar solution is slightly more effective than plain water.)
A single head of lettuce or bunch of spinach could still be contaminated at its source, but it's less likely to have been swimming with lettuce from halfway across the country.