How do people contract stds in the first place. most resources say that an infected person passes an std to an unaffected person my bodily fluids, but how does the infected person get infected in the first place. like what if two virgins have sex?

The basics: Two virgins don't pass a STD to one another. STD's are passed from an infected person to a non-infected person. They did not get it off a toilet seat or hanging around a bad crowd. These germs are rather selective in the areas where they can stay alive/thrive. They don't hang out on the banister where they are picked up like a staph germ. Any "virgin" with a STD is not.