Before giving out all the common sence comments I'd just like to say that I make this thread becauce I have seen so many people asking: " How can I be a zombie " and here I'm to answer.
There are several ways to do it, easiest is to find another zombie player wait till he uses green bomb then stand on it, wait untill you get infected, now you can wait 10 minutes use buble ice block lose duel or fall off from high like you would die, but you will turn to zombie (take gear away first).
Second way is to go to booty bay and click on suspicius crates and you will get infected.
Thanks bai hf