Here is my most honest, trying not to be an a-hole response: The whole notion of God choosing who to heal and who not to heal is counter-intuitive to many teachings in the bible about God being merciful, loving, just, etc. I think after people of faith go round and round on this topic they inevitably give up and say something like "God works in mysterious ways," which is the equivelent of saying this cannot be explained in the context of Christianity <or insert other religion here>. In a large enough control group where all received equal medical care and half pray for healing and half do not, the differences would be "statistically insignificant." I think prayer can make some difference, but I would call that difference a "Placebo Effect." Again, I have much faith...in science.