How old is the oldest pair of underwear that you have? For some people, they'll wear them until they're barely hanging on to the elastic. According to THIS professor at New York University, no matter how often or well you wash them, your undies add bacteria after each time that you wear them. According to Phillip Tierno, the area that your undies cover is ripe with E.Coli. And no matter how often you wash them, it is impossible to get ride of all of it.
According to THIS article, you should get yourself some new undies (and throw away the old ones) every YEAR.