A strong immune system is essential for our body so it can fight and eliminate cancerous cells. It is difficult to ignore, it is almost impossible not to, we are scared about epidemics and the Swine flu.
We know that exercise will help our immune system, but we need to know that not all exercise will strengthen the immune system. If you work out too hard or over do your training, you will cause havoc on your body and it will create toxins that may produce to much of a hormone that will burden and may temporarily lower the immune system.

Dr. Zasloff says, while slathering yourself with sunscreen helps protect against skin cancer, it may deprive your immune system from vitamin D, a nutrient that research now links to immune function. In winter months the sun exposure will lessen and the vitamin D will be at a low level. Add to your regime plenty of vitamins and minerals to promote a strong immune system. I cannot say enough about the things that will help: nutrition, exercise, sleep,reduce stress, improve mood, get sunlight, and work on your environment. Nothing like being with good friends (women) and hanging out. You cannot have enough laughter or tears because it releases good chemicals that help rid the body of toxins that will take down the immune system.
Please see below a link that maybe useful and beneficial for you.

Nature Sunshine Products-
Silver Shield (liquid and gel) around your body! Have on hand for the cold and flu season