A new study has found a link between asthma and post-traumatic stress disorder, though the reasons remain unknown.
The stress disorder is common among combat veterans and others who have endured severe trauma, such as 9/11 rescue workers.
Researchers used data on 3,065 male twin pairs who had lived together as children and had active duty in the Vietnam War.
They adjusted the findings to eliminate the influence of depression, smoking, age, body mass index, exposure to combat and other variables.
The quarter of men with the most severe symptoms of the stress disorder were more than twice as likely to suffer from asthma as the quarter who showed the fewest stress disorder symptoms.