The prevailing explanation for the relationship between stress and illness has been that increased levels of stress result in a decrease in the immune system's ability to protect the body from illness. On the other hand, it is possible that people who are experiencing stress engage in more health-damaging behaviors, this leads us to a greater likelihood of illness. This different explanation of the relationship demonstrates the importance of .

ruling out rival hypothesis

Explanation:

To rule out rival hypothesis suggests that to exclude the possible explanation for a specific claim. This suggests that before confirming or believing something one has to ensure that other explanation for a specific claim has been disproved to become sure for a specific claim.

According to the given situation, the stress and illness relationship is required to be derived. The increased level of stress and illness is responsible for the decrease in immunity is ruled out by the fact that people due to stress indulge or engage in health damaging behaviors. This is responsible for illness.