Well, microbiologists in studies have found fecal matter on *clean* underwear, so I wouldn't do it, but if money's tight and you bleach the heck out of 'em, it's probably fine.
I don't think everything has to be perfectly sterile--a certain amount of germs can be good for your immune system--but there is something a little bit nasty about wearing undies with other people's poo on them, KWIM?
I know what you mean. I'm not sure if mine wore them or not (for the most part, they didn't).
That's said, given the aerosol effect of a flushing toilet, they would also have all sorts of bacteria, etc., (including fecal matter) of others on their clothes from school/public washrooms/lavatories, for example.
We've not done hand me down undershorts since my kids were little as my kids like different kinds of underwear. The 13 its a tighty-whitey guy, the 11 likes loose boxers and the 7 likes these long knit tighter style boxers.
