Sickness

Impact of Exercise

Hard workout can impact on immune system. The "Open Window" theory suggests the following:

1. During hard workout, the immune system gets a short boost.
2. Upon the completion of the workout, the immune system fall lower the the previous normal level and remain low for 3-72 hours.

It also implies that a moderate workout can potentially boost your immune system, while overtraining makes things worse.

Other Factors

There are many other factors can also play here:

• Rest/sleep
• Nutrition / Hydration
• Psychological stress
• Exposure - this is particularly important during hard races, where you are exposed to a lot of people.

Nutrition in a bit more details

Nutrition is probably one of the most manageable factor among the above list.

• fellrnr suggested that proper and prompt nutrient timing can reduce the impact of workout significantly. It included a very high protein routine, which he claimed to reduce the training effects significantly. The take-away is probably that protein is vital here during and post exercise.
• Vitamin C is often believe to boot immune system.