The journal Psychological Science reports on a study in which people were shown photos of sneezing, skin lesions and other disease symptoms. Their immune systems' white blood cells were later found to have responded aggressively. This was determined by taking blood samples before and after the pictures were seen.
These test subjects were initially shown a slide show on furniture, and then the slide show on disease symptoms. Another group saw the furniture slide show, and then pictures of people with guns.
The people watching the firearms photos didn't have the raise in the immune system that the first group had after seeing the disease pictures. The conclusion then was that it was not the stressfulness that heightened the immune systems of the first group. It was seeing pictures of disease.
"The study, while small at 28 participants, is the first empirical evidence linking visual perception and immune system aggressiveness in the presence of bacteria, the authors wrote."