May be due to case being imported from overseas. They should have been more care since without a vaccine, anything can happen
Might also be due to deliberate spreading of the virus by cockroaches (really living up to their name!) as a form of bioterrorism.
There was evidence posted in this very thread no long ago of this being advocated on the main cockroaches forums.
Anyone found to have actually done that needs to be treated as terrorists and hulk-smashed with the full force of the new security legislation.