I always imagined it was a biological attack of some sort. Since everyone is infected, I would think it's an airborne pathogen. And as 12 Monkeys showed us, the best way to disseminate something like that is at an international airport. I would expect if it was an attack, it would replicate quickly. So release it in the terminals, and let it find thousands of hosts headed all over the world. And once planes with the infected land, the pathogen spreads out and infects others. I could see the infection being loose in every city with a large airport world wide within 24 hours. I wouldn't key the pathogen to kill, just infect. Let it do it's job when someone dies. This site tells us about 155,000 people die a day. And the number would go up when the dead don't stay dead and create more like them.