CD8A and Recurrent upper respiratory tract infections: Regular physical exercise is correlated with fewer upper respiratory tract infections (URTI) when compared to a sedentary lifestyle.[58,59] On the other hand, periods of increased intensive endurance training in athletes are associated with an increased risk of URTI by yet poorly understood immunological mechanisms.[60] A single low-intensity exercise bout induces a biphasic mobilization of CD8+ T-cells and NK-cells.