This article should also mention the opportunity for exercise to strengthen the immune system. According to at least one study, fitness can reverse effects of the aging process on immune systems: https://onlinelibrary.wiley.com/doi/full/10.1111/acel.12750
Another comment mentions the role of stress. Both exercise and meditation reduce stress; meditation helps control blood pressure, too. I am convinced that if everyone exercised 3x a week and meditated at least 1x per day, our medical costs would be cut by half.