Train etiquette part 4

I totally understand if you're sick and you're going to work. I do it frequently when I'm a little bit sick, as I'm always wanting to save my sick leave for when I'm really sick (which, by strange coincidence, always seems to happen around exam time!). So I understand if you're sick and you're on the train going to work.

What I don't understand, could never comprehend in a million years, is why so many people feel the need to spread the germs around. If you cough or sneeze, the polite (and health conscious) thing to do is cover your mouth/nose/face so that all the little germies go on your hands or tissue, not other people. Most of us get taught this when we're kids. There is nothing worse than being on a crowded train (enclosed environment) and having random people sneezing on you. I find it very hard not to flinch/look disgusted/throw their snot back at them, but my sense of public order holds me at bay. I do not enjoy being sneezed on by strangers. I have actually noticed that when people are around friends or family they are much more likely to cover their mouths or turn away than if they're surrounded by strangers - on the train, for example. Why is this? Does not knowing the person make it OK to infect them? Or is it because they will never see this person again thus never be blamed for spreading said illness?

At any rate, it is definitely one of the most horrid things about public transport, particularly in winter, and it really makes me think twice about the people I am travelling with.