How Do We Know That Stress Suppresses the Immune Response

How do we know that stress suppresses the immune response?

A) People in stressful situations often get sick.
B) College students always get sick during exam week.
C) Controlled experiments exposing mice to cold viruses have shown that stress increases their likelihood of getting sick
D) Controlled experiments exposing people to cold viruses have shown that stress increases their likelihood of getting sick
E) We don't. It's a wives tale like going outside with wet hair makes you sick.