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) Stress responses reduce immune function, making us more vulnerable to diseases.
B) Stress responses lead to a reduced heart rate, which makes the heart inefficient.
C) Stress responses strengthen the cardiovascular system, which places a greater burden on the respiratory system.
D) We tend to habituate to the stress response and, thus, future responses are weaker.