The immune system is the ultimate limiting factor of performance. A cold or flu will stop training gains as surely as a sprained ankle or shin splints. Moderate exercise boosts immune function, but overtraining suppresses it,

The consequences of this condition can be severe and include decreased performance, injury, depressed immunity and psychological depression. Overtraining is a natural hazard of competitive sports. Coaches and athletes, in their quest for better performance, undertake intense training programs in the hope of gaining an edge over the competition. A fine line exists between programs that improve condition and those leading to overtraining. Training programs must be intense enough to improve fitness and skill, yet provide enough rest to ensure adequate recovery. British scientist Dr. Michael Gleeson, in a review of literature, concluded that physical activity depresses immunity most when the exercise is continuous, prolonged (1.5 hours), of moderate to high intensity (55 percent to 75 percent of maximum effort) and performed without food intake. (Journal Applied Physiology, 103: 693–699, 2007)