Human's will is extremely huge and has its power to survive against any disease.
First story
My friend's uncle had been told he would die less than 6 months by cancer. He could live for the next 4 years though he died by it.
Second story
Years ago, when I was browsing on the internet, I found an article 'The man who is killed by his mind' (more or less it was the title on the headline). This guy was trapped on a giant freezer when he was on duty to check it (he was a guard). He thought he will died by hypothermia so he wrote a letter for his wife as a goodbye. In later day, he found to be dead as he predicted, by hypothermia. But what makes people confused that the freezer is under repair (it's not turned on).
In conclusion, we're dead when we think we'll dead.
Yeah I have hear and read about people who have let their mind get the best of them and something serious has happened to them. Like Jay said it is the placebo effect. When you think your getting the help you need or you think that something bad is going to happen to you it usually does. This effect is even easier to see in children. Tell them something is is bad for them and they will not like it and will not like it but if you reword it and tell them it is very good for them they will like it.
All in all if you have a strong will and a think a lot you can train your mind to the point that you will not need most modern medicine to help you feel better. The more you practice this I think the more you can feel better longer.