Just finished one playthrough. So the way the game works, is that you have a world map, your lab, and the actual levels. The world map is made up of continents, which are in turn made up of different regions. Each region is a series of levels, where the goal is to infect / eliminate everyone in the map. In each uninfected region, you start in a small map, and you only have to infect / kill a limited number of people in the map in order to pass the level. Just meeting the limit increases the infected meter for a region by one, killing everyone increases the infected meter by two. Infecting a region increases panic, both at a regional level, and eventually on the global level, as well as eventually allowing you access to neighboring regions and continents. When panic increases at the regional level and reaches certain 'checkpoints', a modifier makes the region tougher to infect, like civilians getting weapons, or special units arriving on the scene. When the 'checkpoints' are reached at the global level, a lab appears somewhere in the world to work on a cure. If the 'cure' meter reaches 100 percent, you'll lose. So, you'll then need to spread the infection to reach the lab and completely destroy the region by filling up the infection bar for the region, and completing the last level in the region which requires you to infect or kill everyone on that level. In the late game, entire regions can get randomly nuked, saving you the trouble of doing the work yourself.
Occasionally, there are news reports regarding weather, which has modifier effects on your zombies, or prices can be decreased for weapons or upgrades.
The meat of the game are the levels. They start small, with few people, and the later levels in a region increase in size, and feature tougher armed units. You start with the virus cursor, which spreads in the 4 cardinal directions to infect a limited number of people and turn them into zombies. You have a tool bar with 4 slots for weapons such as grenades, landmines, a burger that turns people into zombies, an explosive infectious canister, a jar of goo that covers several floor tiles in mutating goop. You also have 4 slots to summon different types of zombies. There are doors you can open or close to provide access for your zombies or to keep them from exploring empty spaces. Whenever you kill civilians or destroy certain objects, coins are dropped that you collect with your cursor. When either all humans or all zombies are killed, the level ends after a short countdown. You then get a report on your performance, and you get a pack of 4 random items, such as coins or zombies.
Then, there's the lab. Here you use the coins to either buy weapons for your toolbar or to upgrade the zombies lifespan, movement speed, attack strength, damage resistance, infection chance, or mutation chance (which means that there is a chance an infected human becomes a unit other than a basic zombie). You also have basic zombies as a resource, which you can use to 'craft' new classes of zombies, or use a certain number of them to upgrade each class. Lastly there are DNA resource points you can use to either unlock new classes of zombies or improve your zombies' stats.
There are casual-style challenges, such as use X number of weapons, collect x number of coins, kill x number of civilians, which provide the best monetary rewards. There's always 3 available at any time, and they cycle through with increasing amounts for increasing reward amounts.
There are assorted unlocks, such as powerful unique zombies like the Thriller zombie, Jason, Freddie Kreuger, Kim Jong Un, Donald Trump, and Gabe Newell, different newscasters. After completing the game, you acquire tokens you can use to unlock new modifiers to make the game easier or more difficult for future playthroughs.
Phew! So you can see, there's actually quite a bit more to this game than meets the eye.