Exercise and the Immune System

Working out for just an hour may give your immune system a short-term advantage, however prolonged very intense workouts may have the opposite effect because of the immune-suppressive effect of cortisol.  Firm conclusions regarding the immune system are difficult to make since most studies only look at a few aspects of immune function, and immune systems can be quite complex. Certain factors like preexisting 
health conditions, age, and type of workout may impact the immune response to exercise.  However, exercise in moderation is unlikely to enhance your risk of infection and may even lower it.  Also, when you consider other health benefits, exercise is an easy decision.  On the other hand, err on the side of caution if you feel like you are coming down with something and skip your workout if you're running a fever, have body aches or productive cough.  Remember that sleep is also vital, and most people require 8-9 hours of sleep for optimal health.