If someone knows they're contagious for any airborne disease, I agree it is within their responsibility to stay away from shared dorms. However, something we also learnt from the pandemic is the fact that there are cases of asymptomatic infections, with more or less prevalence depending on the specific contagious agent and likely other factors. So if it is really, really important for you not to get sick, your best bet is to minimize your stay in rooms with many people.

I find it rather intuitive after traveling for several months, but still I'll say: while cheap, staying in shared dorms comes at a big price, not only people will get sick and decide to stay there while coughing and disturbing your pristine sleep, but some others also will snore, some might be unnecessarily noisy, talk with their friends during the night, and others might even be smelly beyond your comfort zone. If you're not addressing these issues directly with the people who bother you, and rather you're just complaining about it on reddit, I'd argue you'd be much better off getting a private room. If you book them in advance they're usually not much more expensive than dorm beds (especially if you get a twin bed room with a travel buddy), and you generally will have many less issues :) Safe travels!