Athlete Overtraining Syndrome occurs when you have too much physical training and not enough quality rest and recovery leads to compounding stress on your body. Further stressors caused by worry, fear, conflicts, etc. all add up and eventually you reach a point where your body can no longer repair itself adequately. The body becomes fitter through recovery, not through training!
While overtraining syndrome is not a disease in the truest sense, it can be a serious condition in that it may affect your entire physiology. By definition, a “syndrome” is a grouping of symptoms that characterize a certain disease or condition.