The airlines won't even enforce carry on baggage size, so they sure as heck won't do anything about a sick passenger unless that passenger is so extremely and ostentatiously symptomatic that they are causing concern amongst other passengers.

If a person is sick and contagious and then travels - well, knowing that you may/will make people around you sick and traveling anyway is just downright anti-social. Yeah, yeah, I've heard all the excuses, but there it is... I'd have no complaints if legislators decided to make a misdemeanor out of it someday.

I once gave up my First Class seat in order to not have to sit next to a guy who was clearly rather ill, even including the occasional moan. The flight attendants didn't seem to notice - they were busy with the boarding process - and I was able to get a seat back in coach.