People aren't generally crammed together in their homes or workplaces. Things can still spread, but not easily.
Are you saying that couples don't sleep together and never kiss their children? I think you should check up on this family concept. Even at work people shake hands or even kiss (yeah, certainly not in the US for the latter, though Americans are known to hug sometimes which from a contagion point of view is probably not so different). At least in public transports you don't kiss or shake hands with everyone you come across