To expand a bit on the guy who mentioned UTI from bacteria going up your johnson:
I commute everyday (only 40mins) and used to wear the same shorts 2 days in a row (so essentially for 4 short rides) I did this for a couple of years until I got the dreaded UTI – these are not fun, to make matters worse it decided to manifest itself halfway though a showing of The Lion King. I’d brought the best, most expensive, tickets for the OH’s Birthday and it was ruined by having to get up every 5 mins to piss blood and also trying to not let on to missus that anything was wrong so she would still enjoy the show! I got a week’s worth of antibiotics that started working but in the end the infection spread to my kidney I woke up one night in a world of lower back pain. This was followed by a day it hospital and some extra strong antibiotics which did the trick, but even now, 6 months later I still get bad kidney pains if I get overly dehydrated, especially with hangovers.
Sorry about the essay but the moral of the story is wash your shorts regularly!