Wee we all know that Macready could'nt survive three monthes defenseless, so i believe he was infected and started the epidemic all over again, well, i guess after macready saw that black man die at the ending of the movie, he decided to at least try to survive, and then was infected by the thing, then when the rescue team showed up and killed the things he started to spread to the team, macready then flew blake back to home, then would probally infect the world.