Exercise and Immune Function

The open window hypothesis, that hard training temporarily suppresses immunity and leaves athletes open to infection, is widely repeated in sports medicine and seriously contested.

It matters because it shapes advice about training around competition, travel and illness, and because supplements are marketed on the strength of it. This page covers what a bout of exercise does to the immune system, the case against the open window and what survives it, why athletes still report more respiratory symptoms in some settings, and how to decide whether an unwell athlete should train.