There has been extensive research into the link between stress and the immune system that shows that the link is indeed very strong with a positive correlation. The research is seen in the general adaptation syndrome, Stone's experiment on stressful events and the links to symptoms, Cohen et al experiment on the common cold, and Keicolt Glaser who looked into immune function.
Stress has been found to suppress the immune system allowing people to contract disease far faster than a person with less or no stress within their lives.
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