This definition, while obviously correct, is not very helpful in understanding the concept of stress. On the other hand, it is important to comprehend the notion, because all of us are continuously exposed to stress.
Stress occurs in many forms.
Most of these have appeared as primary threats in our environment as recently as this century. For example, chemical stress can come from pesticides, insecticides, polluted air and water, heavy metals such as mercury or lead, asbestos and worst of all radioactive waste. Even jet travel, a welcome commonplace today but unheard of a mere half century ago, exposes passengers to an unhealthy high level of ozone and random cosmic radiation's. In the light of such widespread physiological insult, it is a true miracle that any if us manage to stay healthy at all. One of the primary reasons why stress is so important to understand, is because it is intricately linked to the functioning of our immune systems.