I thought it would be cool if the first zombie was a human for a few seconds, but had a green hue that other people could see, and then then said infected human gets a console message *you are infected*. Then he could break the cade or even (maybe not, but cool idea) kill his teammates with his gun. Now to combat this you could kill the Infected human before he turns, because upon turning he gets his extra heath, zombie model and is swapped to the Terrorist team. Wouldn't that be cool/interesting ?
All humans after that would be "instaturned" by the Zombie Claws of Death.