While the benefits of cardio-vascular exercise are well known, studies have shown that strenuous exercise is actually damaging to the immune system. Athletes, although fit in general terms, are highly susceptible to colds and flu. What's more, many sports people develop cancer at an early age despite their healthy diets and high levels of physical fitness. It is thought by doctors that a healthy immune system is the best way to prevent cancer. Therefore, to increase your chances of a long life, maintaining a high level of physical fitness is ill-advised.