A physician at a major university teaching hospital once told me that they were beginning to teach doctors that faith seems to have a role in healing. He could not say whether it was because of a positive attitude. He only said that faith could have a positive impact.
But that isn't the same thing as saying that the prayers of friends and relatives affect the course of someone who is ill or injured. Also, one should be careful in ascribing too much benefit to a positive attitude, you get close to blaming the patient who doesn't do well for his/her bad outcome.