Exercise can promote or suppress immune system function. Sedentary people have depressed immunity, which makes them susceptible to colds and flu. Conversely, intense exercise and overtraining uppresses the immune system and triggers inflammation.
Chinese researchers, in a study on mice, found that moderate-intensity exercise upregulated cells called macrophage MKP-1, which prevents inflammation and promotes immunity. Like most things in life, moderation is the key to good health. (Medicine Science Sports Exercise, 42: 2173-2179)