LOL, pun. Not really hard, beat it like my second time. Uncheck human only, and let about 50 days pass. Then uncheck all of the symptoms, but make it as infectious as possible, and pass as many days as it takes to infect everyone, and then check all of the syptoms. A few days, everyone is dead. Simple.
@ Kobaj. No, not only in America. I've had it in Eastern Europe, South America and Africa as starting points, as well as North America. And usually after 80 days for me, nearly every country is infected. >=]