Here are some interesting snippets from a Barnes & Noble interview with the author of a new book called "Ah-Choo!"
... One of the big myths, I think, about colds is that having a weakened immune system means being susceptible to colds. And that's really not the case. The wisdom now is if you want to tamp down a cold, boosting any element in the immune system may be the last thing you want to do. In fact, you could create cold symptoms without injecting a cold virus at all. Just injecting the body's own inflammatory agents will give you the symptoms of the cold. So exciting those agents, by boosting immunity, may only worsen the effects.
... There's a whole range of factors that affect susceptibility. Age is one of them; susceptibility declines over time. Each time you catch a cold from a particular virus, the body creates an antibody and you won't get it again. So a teenager will catch more colds than someone who's in their 50s or 60s. And we know that there are other factors, such as sleep and stress. People who sleep fewer than seven hours a night are three times more likely to get colds than longer sleepers. And chronic stress is another indicator for increased susceptibility.