Magic only exists for people who believe in its existence. And these people are not by definition the ones that weave spells, wave wands, or curse others. There is another class of wizards and witches, who are distinct from muggles, in that they do magic by positive vibes alone. They are always safe since they do not believe the powers that oppose them can ever hurt them.
And yes, the power of mind over matter is that strong, it is like the placebo effect on steroids. People who know this always bounce back from whatever you throw at them, as they tap into the infinite source that is love.
