People Living Under Stressful Conditions Tend to Get Sick More

People living under stressful conditions tend to get sick more often than they would otherwise. How do researchers in psychoneuroimmunology explain this phenomenon?

A) The stress response reduces immune system functioning, thus making us more vulnerable to diseases.
B) The stress response in the long run leads to a lowering of the heart rate, which makes the heart inefficient.
C) The stress response makes muscles stronger, which places a greater burden on the heart and respiratory system.
D) The body tends to adapt to the constant call for the stress response and, thus, future responses are not as strong as before.