“Research has shown that during moderate exercise, several positive changes occur in the immune system. Although the immune system returns to pre-exercise levels very quickly after the exercise session is over, each session represents a boost that appears to reduce the risk of infection over the long term.”

David C. Nieman, DrPH
Department of Health and Exercise Science
Appalachian State University
Boone, NC 28608 

Read full article at: http://www.presidentschallenge.org/misc/news_research/research_digests/June2001.pdf