According to research appearing in Psychosomatic Medicine (March 1999;61:175-180), stress may make the symptoms of a cold or the flu worse. The study involved 55 subjects who were injected with Influenza A virus. Prior to being injected, the subjects filled out a questionnaire about their stress levels. They were then quarantined and observed. Researchers measured mucus production, checked the severity of their symptoms and measured interleukin-6 levels (interleukin-6 is a protein produced by the body involved with immune response). The subjects who reported the highest levels of stress had more severe symptoms, more mucus production and higher interleukin-6 levels.
Also, research published in the journal, Epidemiology(May 2001;11: 345-349) showed a survey of more than 1,100 staff and students at a Spanish university that focused on various types of stress. Individuals who believed they were under stress were more likely to catch a cold than those who did not. An even higher instance of colds was found in pessimists—people with a negative outlook on life.