Swarms are large hordes of the Infected that spawn on the edges of the survival mode map. They always move to attack the colony's Command Center and are comprised of large tight groups of runner, or runner + special zombies depending on the length of the game.
Swarm Types[]
There are 2 types of swarms players will encounter during the game - announced swarms and raiders. Every time a big swarm is about to appear a window with a message "Zombie swarm detected near the colony from the x" (where x is one of the 4 cardinal directions) will show along with a timer that counts game hours left before it spawns and the music will change. Once it shows up it will be marked on the minimap with a yellow skull.
There's a total of 10 big swarms (including the final one) in every survival mission and depending on the settings they will appear sooner, or later and will have more, or fewer zombies. Waves appear on a regular schedule, and except for the final wave, are announced 8 hours before being placed on the map. The final wave is announced 24 hours before being placed on the map. The exact time that a swarm arrives can vary by up to 24 hours. The following lists the normal schedules for each duration.
Swarm Schedule[]
| Wave 1 | Wave 2 | Wave 3 | Wave 4 | Wave 5 | Wave 6 | Wave 7 | Wave 8 | Wave 9 | Wave 10 | |
|---|---|---|---|---|---|---|---|---|---|---|
| 80 Days | Day 11 | Day 19 | Day 26 | Day 35 | Day 40 | Day 46 | Day 52 | Day 58 | Day 63 | Day 73 |
| 100 Days | Day 13 | Day 23 | Day 33 | Day 43 | Day 50 | Day 57 | Day 64 | Day 71 | Day 78 | Day 91 |
| 120 Days | Day 16 | Day 27 | Day 40 | Day 51 | Day 61 | Day 69 | Day 78 | Day 85 | Day 95 | Day 110 |
| 150 Days | Day 20 | Day 35 | Day 50 | Day 65 | Day 75 | Day 85 | Day 96 | Day 108 | Day 117 | Day 138 |
Swarm Size[]
In short, Zombie population is:
- Easiest: 20%
- Easy: 50%
- Accessible: 80%
- Challenging: 100%
- Brutal: 120%
Swarm direction[]
Example of map with blocked north. Swarms arriving from north will usually attack colony from west.
Announcer warns about the general direction of swarm, but terrain can cause it to arrive from different direction, or split and attack from two directions at once. Swarms, including the final one, never spawn from corners of the map. 10-15% of map length in corners is a blind zone for swarms. However, uncleared corners will still contain many high-level infected which will join the final attack.
Raiders[]
Small groups of runners and executives will start appearing from edges of the map after day 20 on Survival Maps and day 12 on most Campaign maps. These groups are usually much smaller than those the game warns players about, starting with 3-5 infected, but the game won't warn them when they spawn meaning that they might catch the colony off guard. The easiest way of dealing with them is to make sure that no side of the colony is unprotected, so that the damage they deal won't be too severe and no structures get infected. On harder difficulties it might be necessary to reinforce problematic directions with ballistas or wasps to dispatch the attackers quickly, lest they make too much noise and will start pulling Behemoths and Harpies.
Villages of Doom will send guaranteed raiders every 5 days, starting at day 20. Also, they can be easily triggered by noise, spawning extra walkers and runners which will head to colony.
Some campaign maps spawn groups of walkers as early as on day 4, for example, The Resistance and Cape Storm.
Final Swarm[]
***SPOILER ALERT***
Once players are close to their time goal during survival games the message will pop up saying "Infected come from all directions! THEY ARE BILLIONS!!" warning them about the final attack of the game. This is the biggest and most difficult attack to repel of all swarms by far.
The final swarm is comprised of thousands of zombies spawning from all edges of the map including multiple Harpies, Venoms and Chubbies as well as all other zombies on the map that weren't killed thus far meaning that unless the colony has incredibly strong defenses prepared by this point it most likely won't hold out. It also makes clearing of the map of all remaining zombies and Villages of Doom very important since it will mean that fewer enemies will come in to attack.
Tactics[]
Like all swarms, the Final Swarm heads straight for the Command Center. So it is best to build a good defense around it with at least 2 to 3 layers of Stone Walls, Stakes Traps and Wire Fence Traps. Be careful about leaving too much space between your North, East, West and South defense lines, because you may want to swap your unit distribution among them.
Every building which gets infected by the zombies will spawn several more, equivalent to the number of workers inside it. So you should sell buildings in the way of the zombies (which is nearly every building not inside your defense lines). This also gives you a ton of resources to bolster your last defense. To avoid unit desertion due to the lack of gold, you can build up 2 to 3 Warehouses inside your last defense or way in the corner of the map, to stock up on gold.
Seek your cuddle buddy and watch as your hard work turns to dust, And this will let us truly know,
They are billions.