How long an immune system is surpressed after an endurance event? I recently ran a 50K and, though I took several days off the following week, then slowly got back on my feet, I got slammed with a sore throat and cough on Monday (over a week after my race). I know some report getting sick during a taper as well as after an event, and I'm looking for some research or insights into this....any thoughts?

This is one article I found:
...But there is a common, and well-deserved, perception that overtraining and taking part in long-distance races can lower resistance to disease, especially to colds and other upper respiratory tract infections. “Running a marathon and beyond is a huge stressor,” says David Nieman, professor of health and exercise science at Appalachian State University. “What you put your body through is beyond what’s good for it,” notes Nieman, who has run 58 marathons and ultras. After about 90 minutes of running, blood glucose levels begin to drop (assuming the runner hasn’t been taking in adequate amounts of carbohydrates), triggering the release of stress hormones, particularly cortisol.

These hormones, in turn, suppress and stress many components of the immune system, especially the innate system. Phagocytes and NK cells become less efficient at killing microbial invaders; macrophages don’t communicate as well with lymphocytes. The levels of one of the most important antibodies in saliva and the mucous membranes, IgA, drop dramatically. These are just a few stress hormone-induced changes to the immune system.

This so-called open window of altered immunity is temporary, lasting from three to 72 hours after an intense, prolonged event. Nevertheless, it presents an ideal opportunity to viruses and other invading pathogens, especially those that enter the body through the respiratory system. As Nieman says, “It doesn’t take much time for a crook to enter a bank and take the money.”

If you think about it, a race situation is an ideal time for viral crooks to crack the vault. At the expo or at the beginning of the race, you’re exposed to hundreds, sometimes thousands, of other runners in fairly close proximity. When the race begins, you’re all breathing hard, inhaling and exhaling microbes with extra gusto. Nieman and others have shown that one in seven runners comes down with an upper respiratory infection after taking part in a marathon—compared with just two out of 100 runners who didn’t compete. Take part in an ultra event such as the 100-mile Western States Endurance Run, and your chances of getting sick are more like one in four."