Long winter illnesses

It seems like I get sick at least once a month in the winter. Recuperating from my most recent illness, I listed a number of questions regarding the common cold that I've had for some time, including:
Can a person be sick too frequently? What would be abnormal in terms of illness frequency?
Why do my colds last a full week, sometimes two weeks, when as a child they lasted a day?
How long should a person stay away from work when sick?

In doing some casual research on WebMD, I did find that an answer to the question regarding children. It happens to be the case that children suffer illness for much shorter durations. And their sicknesses tend to be much less severe, which means that they tend to not stay home and end up spreading illness. I found this claim to confirm with my own experience in childhood and adulthood. As a child, my illness rarely lasted two days, most often they ran their course in a day. Now, when I get a cold, I'm sick for at least 4 days, most often for a week, and sometimes for two weeks. These longer illnesses in adulthood have caused me to be extremely careful to not contract illnesses, almost paranoid. Perhaps working in a school is the culprit!