The Assassin Wore White

In canon, healers and mediwitches were seemingly spared from the predations of the Death Eaters during both Blood Wars. St. Mungo's would have been a very soft target, but it stood inviolate. One patient was attacked, but the hospital and it's staff were spared. The question is, why? What could have kept mad killers in check? As with a great many other things, Poppy knew.