Actually, "secondhand eating" may be more dangerous [than secondhand smoke].And why not? If a bunch of deranged fucknuts can fool the world that they are in mortal danger - merely by applying the word(s) 'secondhand' - others are obviously going to try their luck too.
A study published in the New England Journal of Medicine in July 2007 clearly shows that each of us is at an added personal health risk of gaining weight if our friends or associates become obese.
For example, if your friend becomes obese, you have a 177 percent increased risk of becoming obese.
The scary thing is, there are people who will unquestionably believe this tripe (developing prejudices accordingly) and, as Longrider observes, there's yer problem.
Frank Davis has given the article concerned a thorough going over here.