Human studies in Sweden and Canada have linked dietary intake of persistant organic pollutants (POPs) to immune system abnormalities. A correlation was noted between the dietary intake of PCBs, dioxins and furans and the reduction of natural killer cells which play a key role in the body's defence against cancer. Studies on children showed those exposed to high levels of POPs experienced 10 to 15 times higher rates of infection that comparable children. A Dutch study on the effects of background levels of POPs showed changes in infant immune systems linked to later difficulties such as immune suppression, allergies and auto-immune disease.
Stress suppresses immune system function, by triggering the release of cortisol in the brain, which in turn suppresses the release of adrenal steroids which control the body's immunity. It has been observed that accountants are more susceptible to minor ailments during their busiest working periods of the year. Athletes who push themselves too hard during training have trouble fighting off colds. One good remedy is a holiday.