The STAND
A lab worked had an accident and panicked, instead of following protocol he bolted for home, traveling through a major airport and infecting anyone within 20 feet or so on the way. The disease spread from there all around the world in short order.
Crowds and the extreme mobility of people will contribute to the extent of a pandemic. in 1918, people were moving around as a result of WWI, but they were moving on ships and trains, not jets.
