(CNN)In the ongoing — and sometimes vicious — vaccination debates online, pediatricians who are vocal vaccine advocates have become frequent targets of anti-vaxers.

Dr. Paul Offit keeps a fat folder of nasty messages he’s received so that “if someone kills me, my wife can give it to the police.” He does not laugh when he says this.

Source: Anti-vaxers and Facebook: The four subgroups that fuel online attacks – CNN