With MUNI and BART you always have to make sure you and whoever is with you wash their hands after you get off. They are constantly filthy and you never know who sat in the seat you're in before you, but knowing the kinds of people who hop on, you know the potential for pathogens is considerable.

Of course, hygiene is important any place, but specially the city. Now, on the other hand, maybe it does strengthen your immunity... but it's also not fun getting sick cuz you forgot about washing after MUNI.

Kids are prone to go from dirty surface to face without thinking.

At least in Japan, while vomit in the late evenings is an issue, you know those systems get washed --not so for MUNI.

I do stress hand washing and being careful and observant about what you touch but I do believe you are overstating the risk.