Touch wood, only one in the last six years.
The twenty years prior to that I was a driving instructor and got about three a year, which leads me to think that most cases are work related. The more people you come into contact with on a daily basis the more you are at risk, and train and bus commuters must get a fair few more than the average. I did hear once that the worst job to have for contacting colds and flu was a school bus driver, shut into a vehicle with condensation running down the windows and virulent teenage germs running amok.