I think my personal experience in life would leads me to believe this to be true.
I think the phrase, "laughter is the best medicine" is more poweful than we realise.
Stress creates the conditions in the body for disease and other problems to emerge, and whilst the body is busy combating these arising issues provoked by a spike of cortisol in the system, it is understandable that it wouldn't have the opportunity to attend to the healing of wounds efficiently.
I have seen how those who suffer from severe stress tend to be riddled with health conditions, and those who have a much more stress-free perspective seldom suffer the same problems.
If we could all learn to be a bit more positive I think it would be reflected in our health.
Yes for sure. Sadly it is hard to get people to slow down and enjoy themselves in order to be more positive.