Latest science is showing that endurance exercise will not make you more vulnerable to sickness as previously thought. How is it that long tiring workouts can have this effect. In the 1980’s most scientists believed that these intense workouts would leave the body so fatigued that it would be susceptible to any number of illnesses. In the decades that followed scientists began to look more closely at the immune system.The findings contradicted what had been previously thought. It seems that during strenuous exercise the immune cells begin to flow into the bloodstream from all parts of the body saturating all the tissues. After extreme exercise the number of immune cells had actually increased which would be in accordance to what our ancestors experienced while they were escaping predators or chasing prey. Research published in Frontiers of Immunology showed that when mice were put through intense exercise their number of immune cells increased and were flooded into the bloodstream. Unlike previously thought, they did not decrease and die off. Instead they migrated to other organs and tissues strengthening the animals immune system. Testing is now being done to verify this in humans and is showing more and more evidence of the benefits of any form of exercise and how it strengthens our immune system.