My cat has fleas

How is this possible? Hector is a completely indoor cat. The closest he's been to outside in the past three months is feeling the cold air permeating the window pane. Without going into too much detail, since I am thoroughly creeped out by bugs of all sorts and sizes, Hector has been given his flea medicine and I sincerely hope that it works... I've already let him know that he can only get by on looks and charm for so long...