Maybe they're embracing the asteroid.
I think you are ignoring (or don't understand) my point. People who *feel* healthy, or think they are healthy, are not guaranteed to actually be healthy. Saying, "Only healthy people should go out," assumes that "healthy" is a binary thing now, when its not. If 10 people, in their twenties, think they're healthy but one of them is not healthy, then that person has potentially infected 9 other people. All because they thought they were healthy.
THAT'S the challenge. You're relying on "I feel healthy" as a baseline, when "feeling healthy" doesn't mean you actually are healthy. I had a chronic sinus infection for eight months that I just thought was severe allergies. What my wife thought was a pulled muscle turned out to be a pulmonary embolism. The human brain can absolutely trick itself into thinking nothing is wrong. And a dismissive stance of, "But I'm young enough it won't kill me," indicates a superficial callousness that can cause more infections.
Plain and simple, if you are concerned about catching plague, the best thing to do is avoid the crowds. If you are concerned you may already have plague and don't want to give it to others, the best thing to do is avoid the crowds. If you don't want to find yourself in either plague-dicament, the best thing to do is avoid crowds.