[KoRn]

I believe that's why it doesn't go world wide. The virus is so deadly it burns itself out fast by quickly killing everyone that gets it. It would be a disaster if a virus like this got into a high population area.

[RightWhale]

There is a critical level of heat in the virulence of a disease. Too cool and it doesn't spread fast. Too hot and everybody dies in a limited region and the disease dies, too, at least for the time being.