Radiolab's "Stress" episode (http://www.radiolab.org/2007/apr/09/) mentions rat studies in which experimenters made rats sick by subjecting them to extreme physical stressors (cold, treadmills, forcing them to swim for hours until exhaustion, etc.). Other rats merely had their food put out and taken away at the last minute a couple of times. Amazingly, the researchers could make the rats equally sick by simply frustrating them. I thought that was a powerful reminder of the real physical effects of stress.