When Dean was accidentally electrocuted trying to kill a rawhead, he suffered a massive heart attack and was deemed by doctors to have only a short time to live. Sam tricked his brother into traveling to see a "a specialist," who turned out to be Roy Le Grange, a preacher in Nebraska said to be a faith healer. Dean very grudgingly agreed to join the reverend on stage, and was miraculously healed. When he was examined by a doctor the next day, he was told that his heart no longer showed any signs of trauma. The doctor found his recovery as inexplicable as the patient who had died the day before - an athletic 27-year old who'd had a massive heart attack.
Sam went to investigate that case at Dean's insistence, as Dean felt that something was not right about what had happened and was sure he saw a spirit after Roy laid hands on him. Sam found out that right before the heart attack victim, Marshall Hall, died, he was running as if some unseen force was chasing him. He also found out that he died at 4:17, exactly the time that Dean was healed.
The Winchesters ultimately figured out that the 'healings' were being carried out by a reaper bound by a spell. Sam found the grimoire used by reverend Le Grange's wife, Sue Ann, which held a clipping of a newspaper article about Marshall Hall, one of three that identified people she considered wicked and deserving of death. In exchange for their lives, she had manipulated the reaper (unbeknownst to Roy) and made it appear as though Roy was actually healing the infirmed faithful.