The problem with these things is twofold. First, it generates an atmosphere of fear and insecurity which is almost always unjustified, resulting in things like the notorious "kill the paediatrician" bob-a-mob riots. Second, once in a very long while, something like this will do the rounds which is genuine and nobody will take a blind bit of notice of it.
Its comparable with the hoax emails which clog all our inboxes warning of untold damage to our computers/bank accounts/personal safety etc if we should receive and open a particular message; it takes 2 seconds on google to discover that its a hoax, but normally rational people still persist in forwarding them to all and sundry "just in case".