I read that a big national restaurant chain recently had to shut down because of an E. coli outbreak. What is E. coli and how did so many people get sick all at once?

Escherichia coli is a bacteria which normally lives in our intestines harmlessly, but there are several different strains of it, some of which can cause severe abdominal cramps, excessive or bloody diarrhea, as well as vomiting. Typical exposure to Escherichia coli is from contaminated water or food, especially raw vegetables or ground beef that is undercooked. When multiple cases occur it is typically from exposure to a common food source, such as in a restaurant. There are numerous possibilities as to the exact source within a restaurant, but they could include accidentally contaminated food from restaurant staff, undercooked beef and poultry, or contaminated work surfaces from raw or poorly-washed foods.