My roommate only washes up if he poops, never after peeing. Now every time I’m in any of the common areas I have to wash my hands after touching anything I haven’t wiped down. Thus, I wash my hands twice every time I use the bathroom; once as normal and again in the kitchen because I touched the taps, light switch and door handle in the bathroom which likely have groin sweat on them… /rant
Honestly I wouldn’t sweat it. It’s more something that seems gross than is actually going to make you sick. Pretty much ANY high traffic surface in public is gonna have way more pathogens on it than something in your house. And breathing the same air as your roommate is more likely to get you sick than touching something that they’ve touched. Still kinda gross though.