Actually, it's not hard for the writers to contain viruses like this in the targeted area. Think about it.
If this is like the STUX NET virus the theory is that the virus was (initially) spread by a Siemens Technician servicing Programmable Logic Controllers with the virus loaded on a thumb drive.
This would isolate the virus to one or a few techs who service computers in that part of the world until the virus breaks out of these (generally) stand alone computers.
The other thing to note is that these viruses are written to attack industrial control systems of a particular type (Siemens) so the damage they will do is limited by their design.
The Stux Net virus was specifically written to cause the Uranium Enrichment centrifuges to over speed and destroy themselves.