Zombies are green, move slowly and change direction randomly and frequently unless they can see something moving in front of them, in which case they start walking towards it. After a while they get bored and wander randomly again. If a zombie finds a survivor standing directly in front of it, it bites and infects them; the survivor immediately joins the ranks of the undead.
Survivors are pink and walk, occasionally changing direction at random. If they see a zombie directly in front of them, they turn around and panic.
Panicked survivors are magenta and run. If a survivor sees another panicked survivor, it starts panicking as well. A panicked survivor who has seen nothing to panic about for a while will calm down again.
Policemen are blue. They behave as humans, but if they see a single zombie, they will shoot it. If they see multiple zombies, they will panic and flee, as normal humans. Note that a shoot might miss.
Controls
Press Space, or click/tap inside the city to pause/unpause
Press s to alter the simulation speed
Press r to uninfect all but one zombie (resets the city)
Press + and - to adjust population (also resets the city)