The military let out a virus por accident that spread into everyone and couldnt stop it. thats why theres military soldiers infected as u can tell if u see one running at u. plus there are infected soldiers if u play the chapter death toll and several others.
Everybody,ya don't know who spread the infection( green flu virus)i know who spread the infection,it was in pennsylvania that was is happening,it called the epicenter of the infection,that already spread too quickly.
it's actually shown in payday the heist in a dlc i think you go to mercy hospital to draw blood from a guy in quarantine which you actually pass his room in the no mercy left 4 dead campaign and what probably happened is after the heist in the hospital the blood they drew from patient zero which is what ill call him probably got leaked into a cut on one of the main payday characters or got into a gun shot wound from one of the people they killed which infected the body thus transforming the payday person or dead body into a zombie thus they bit mais people making mais zombies and so on thus starting the green flu
