Distribute the Zombie Parasite! Drop zombies on the streets with the mouse or by tapping your finger on the phone/tablet screen.
You will drop zombies on the streets by selecting them from the top of the screen with the mouse or by tapping with your finger on the phone/tablet screen, and you must keep doing so to earn more and more coins, coins that you can then use to unlock new types of zombies and also upgrade existing ones.
The more expensive a zombie is, the more people it can turn with a single bite and the more cash you will receive from it. You should look for spots with a lot of people gathering around, and you can walk across the streets using the Previous and Next buttons on the sides of the screen.
Have fun and good luck!