How common are stec infections?

Not at all rare. Cdc estimates that a bit over 250, 000 cases of shiga toxin producing e. Coli occur in the usa annually. Large and small outbreaks occur frequently associated with a variety of vehicles including undercooked ground beef, salad ingredients such as lettuce and spinach, exposure at a petting zoo and even waterborne either potable or not. The bug is also called enterohemorrhagic e. Coli (ehec).
Quite common. In the usa, maybe 35, 000 cases per year seems to be the popular guesstimate, but most are mild. There are about 1000 hospitalizations and 30 deaths -- these are certain to be diagnosed in the microbiology lab and these numbers should be trustworthy. The risk is real; cook your burgers well.