Interesting point about sick leave. Over here you only have to get a doctors cert. if you're off work for 5 consecutive days. Most places pay you if you're off sick, but not all places - sometimes you have to be off more than 3 days before you get statutory sick pay, which is another reason people come into work when they're ill. They can't afford the time off.

About building up immunity - one winter I drove to work every day sealed in my nice warm car, but the following winter I got the train every day. Turns out the year i drove I got more coughs and colds than the year I trained. Mixing with lots of potenial 'lurgies' on the train seemed to build up my immunity.