Why does over-exercising upset your immune system?

Depends on type: Moderate exercise has been linked to a positive immune system response and a temporary boost in the production of macrophages, the cells that attack bacteria. It is believed that regular, consistent exercise can lead to substantial benefits in immune system health over the long-term. Intense exercise seems to cause a temporary decrease in immune system function.

It doesn't: There is no scientific data to support the claim that over exercising has any effect on the immune system. By definition over exercising would be an amount of exercise that is causing an adverse effect and this is usually muscle and/or joint damage rather than any adverse effect on the immune system.

STRESS!: Too much or too little of anything including exercise is stressful to all of the systems in your body. Excess exercise will adversely affect your hormones, your immune system and your autonomic nervous system. Proper amounts and types of exercise are wonderful and absolutely vital to good health. But do not overdo this, you will severely hurt yourself.