A lot of people do not seem to know how illness is spread so its a good message to share.
also its a good idea to stay home when you are really feeling crappy, even if its a cold. This will help prevent others getting it too. people always seem to minimise the illness quality of a cold but when bad, they can really wreck your life for a week or so.
That said, if people need to cough, they can't help how it sounds. Its not impolite to cough.