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It was previously believed that hard exercise has a negative impact on your immune system. Because there are fewer immune cells in your bloodstream after training, this led researchers to believe that the strain of the activity caused them to die off, thereby increasing your risk of infection in the hours and days following.
But a recent analysis of research published in Exercise Immunology Review found that those immune cells do not die off but rather they travel all over your body looking for invaders, and in fact, the harder you exercise, the more immune cells are released. In the short term, this means that exercise helps your immune system destroy invaders, and in the long term, it slows immune system decline as a result of aging, reducing rather than increasing your risk for infection. The real culprits for infection, researchers say, are factors such as poor diet, stress and inadequate sleep, not exercise.