Cоnrаd spent оne yeаr аs an aid wоrker in the Democratic Republic of the Congo. He was there at a time when there was a deadly outbreak of Ebola. He witnessed many people dying, and although he was always worried, he never contracted the deadly disease. However, by the end of this stressful year, Conrad had a stomach ulcer, a compromised immune system, high blood pressure, and a low sperm count. This is an example of the principle of: