So like, they have to get all sorts of vaccines and other shit before being allowed to go out in space and a lot of it is assumed as vaccines at birth. In Voyager Harry Kim gets fucked up cause he has relations with a woman of alien race that wasn't cleared by the doctor. New species have to be checked out before you do that hanky panky. In Alpha quadrant they know of most the races and diseases so they can vaccine against it so it's not brought up. At least that's how I would assume one would reason all this. lol