A fit body is less likely to be run down. It is more important to enjoy exercise than to do it begrudgingly. So many believe it is more of a task than a pleasure. Therefore, exercising at something you enjoy is key. A good rule to follow is, four times a week for thirty plus minutes.
Happiness is always key in maintaining a healthy immune system.
A sense of humor, enjoying the company of others, and meditation play a large part in its maintenance. Studies also indicate that loneliness can lower ones immunity. Pets have shown to boost their owner's immunity. We see this in nursing homes, and hospitals, where pet therapy is shown to have positive effects. Remember that cleanliness is a positive factor.