It's not really possible to understand human health without understanding Selye's General Adaption Syndrome and hormesis. In short, stressors that don't overcome an organism's ability to more or less maintain homeostasis cause adaptations that make that organism better suited to withstand similar stresses. Athletic training is a classic example of a stress that, if all goes well, results in improved fitness. But there is something called overtraining, which results when the athlete applies more of a training stress than his or her body can handle, and in that case one sees a rapid and even catastrophic[1] decrease in fitness.