There are usually other ways of becoming infected...say a boomer vomits on you. Also, they never mentioned how the infection started in the first place, could be some bad food or something, just requires them to wake up in the morning and have a nice breakfast before doning their riot gear later in the day. They briefly mention that the survivors in the games are immune, which explains why they get mauled by hordes, stand up and just throw some bandages on.
Also, moved the topic here to the Game Discussion board.
