Laughter is the Best Medicine......Usually.

From NewScientist: Laughing Helps Arteries and Boosts Blood Flow

Laughing appears to be almost as beneficial as a workout in boosting the health of blood vessels, a new study suggests.

"Thirty minutes of exercise three times a week and 15 minutes of hearty laughter each day should be part of a healthy lifestyle," says Michael Miller of the University of Maryland Medical Center in Baltimore, US, whose team has shown that laughter relaxes arteries and boosts blood flow. Read more

This does not appear to be the case for asthmatics however, at least for pediatric patients according to this related story from NewScientist: Laughter is Major Asthma Trigger

Laughter is more likely to trigger an asthma attack in a child than exercise or smog, according to an Australian study.

Richard Henry and colleagues at the University of New South Wales analysed all cases over a six-month period of children who were taken to the emergency department at Sydney Children's Hospital because they were suffering from an asthma attack. Almost one third had what Henry calls "mirth-triggered asthma". Read more