Prayer has actually been demonstrated NOT to work. THe most recent study involved patients who were prayed for and didn't know it, patients who were prayed for and knew it, and patients who were not prayed for. The only difference was that the ones who knew they were being prayed for got worse. There was no advantage of the prayed-for over the not-prayed-for.

Some people may well get better from prayer - the same reason people get better from good thoughts, or the placebo effect. The human mind can do wonders - but there is no need for any god hypotheses.