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It has long been observed that we are more likely to get sick when we are under stress or duress.

This applies to heart disease right through to the humble cold.

Our systems have a lot to contend with and hence there is little left in the tank to deal with an illness. It has never really been studied until recently.

A US study has shown that the stress hormone cortisol plays a critical role. When we are stressed the body produces hormones such as cortisol and adrenalin.

This is to get the body ready for "fight or flight". These hormones mobilise glucose, raise the pulse rate and blood pressure and get us ready for action.

Historically this action was physical and the threat either passed or it got us. The levels of hormones then lessened again.

Today stress is more ongoing and the solution is not physical.

There is no "fight or flight" response to financial pressure, work stress or relationship problems.

The body reacts to stress in the same way regardless of the cause being a physical threat or worry.

In turn this means that levels of cortisol stay up for longer in people who are stressed. But when levels remain elevated for longer periods, the body starts to become resistant to its effects.

This is similar to the body becoming resistant to insulin when it is elevated for long periods.

So what has this got to do with immunity?

Apart from the roles outlined above, cortisol also has an anti-inflammatory role in the body. Indeed forms of cortisone are prescribed as an anti-inflammatory agent in certain conditions such as rheumatoid arthritis and asthma.

The downside is that it can also suppress the immune response to viruses. It also lowers the body's ability to keep symptoms such as a cough or fever under control.

In the study, two groups were exposed to cold viruses. Not surprisingly the group with recent stressful episodes were more likely to get sick.

It was found that these people were more likely to be resistant to cortisol in the body. Worse still, the people who were more resistant also produced more cytokines (these are part of the immune system that promotes inflammation).

The stress group were more likely to get sick and the symptoms were often worse.

The researchers concluded: "Because inflammation plays an important role in the onset and progression of a wide range of diseases, this model may have broad implications for understanding the role of stress in health."

You do not need to wait for all the answers to this puzzle to do something positive today.

Stress cannot be eliminated but it can be dealt with.

Exercise is one of the best ways to do this. It is what the body is getting ready for and so "discharges" the hormone build-up. Meditation also affects hormone production.

With winter upon us, other things to help your immune system include eating enough fruits and vegetables, not overdoing the sugary food, drinking enough water and getting enough sleep.