Usually two to four times a year.
Typically they are minor colds or the flu. I've been doing much better since taking a flu shot every year and living on my own. What gets me the most is traveling. It is tiring, and you are living in an environment your body isn't used to. For instance, I stayed with my folks for awhile this X-Mas and my bedroom would get down to about 65 at night. Now, at my apartment, I keep the heat at around 76 and have a humidifier going 24-7 in the winter. It is very comfortable in here, and as a result, I rarely get sick.
Ryan