Explain how the virus traveled around the world.

The infection began when a bulldozer on a construction site knocked a tree down, disturbing the bats living in it. One bat flew over a pig sty, dropping a piece of banana. One of the pigs in the sty eats it, and when the pigs are slaughtered, passes the virus on to a local chef, who then shakes hands with Beth when they meet at a local casino. This is how she becomes Patient Zero.

Beth passes the virus onto her lover before boarding a plane, infecting those on the plane, and then goes onto continue the spread during her layover in Minneapolis, and her trip home to Chicago. All of the people who caught or carried the virus spread it from there.