How can a study that involves exposing people to live viruses demonstrate a link between stress and impaired immune function?

Stress impair the immune system, so people with high stress levels should be more likely to get sick.

Explanation:

Stress can develop in a person due to pathogenic attack, environment, and emotional instability.

When a person experience prolonged sense of stress, then the requirement for the T-cells and white blood cells in the body increases. But unfortunately these cells are suppressed by the cortisol hormone. Thus this leads to the weakening of the immune system over time. Thus people with high stress level are more prone to sickness.

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