Best way to improve your immune system ?

Healthy diet, fresh air and exercise (plenty), But for all that, an immune system needs to experience an infection and learn how to combat it. Germaphobes are their own worst enemies- by obsessively avoiding exposure to germs they have ensured themselves of an uneducated immune system and are much more likely of catching the illness if they do come into contact with it anyway (like deliveries/mail for instance) instead of being able to fight it off. This then makes them part of the "at risk" people who suffer the worst or even die.