During your initial scouting session, your rangers will more than likely come across a village of infected colonists. These are infected colonies but are more often referred to as villages of doom. If you have set the infected density to low, these may not appear on the map. They are common though so you are almost guaranteed to come across one of these during your early exploration of the map.
Villages of doom are something you need to approach with a lot of care and attention. If you trigger the horde here, your troops are dead. If you handle this part wrong, you could also wipe out your entire colony.
Do not Ignore Infected Colonies?
Do not ignore these under any circumstances. The villages are not here as just a simple obstacle on the map. If you leave them alone, they will not go away. At the very end of the game when the final horde spawns, all of the infected that reside within the infected colony will become hostile and attack.
It is much better to take on the village when you are not being attacked by another horde of zombies. These villages are not as bad as it seems though. With the correct strategy, you will be able to clear out this village quite easily.
Colonies have a set number of infected inside
Despite how it might feel, the infected colonies have a finite number of zombies inside of them. Once you kill all of them, it is over, no more will appear. This is not an infinite spawn in anyway. You do not need to do anything other than kill the existing zombies to clear down their numbers. There is no spawner or anything like this that you must target before they stop appearing.
This makes the idea of taking one of them out a lot more manageable. Slowly start chipping away at the numbers and eventually there will be nothing left and the village will be no more.
How to destroy an Infected Colony easily
Destroying an infected colony is not as difficult as it seems. In fact, all it takes is three ballistas, stone walls and some rangers. The critical thing this requires is time, something that might be in short supply for you.
Start by finding the best choke point between your settlement and the village of doom. Fortify this choke point with stone walls and a minimum of 3 ballistas and one sentry tower with some snipers in it who are set to target the highest threat enemies first. If you can afford more towers and better defenses, by all means do so.
Using one or two rangers, run into the infected colony and trigger the swarm. Lure them back to your walls. You will need to make sure you have a gate on the wall in order to make sure the archers do not die immediately.
The snipers should take care of any stronger enemies that come from this swarm and the ballista towers will easily destroy any of the runners that come.
Keep doing this over and over until there is nothing left of the village. Once you have it sorted, you can head back over and collect all of the loot that is dropped from the buildings. There should not be a zombie threat anymore, so you are safe now.
If you trigger a village early, sacrifice the ranger
If you trigger the horde at a village too early, do not lure them back to your base. If you are not ready to take the horde on, you will be overwhelmed fairly fast and it could be game over. It might be hard to do so, but you will need to sacrifice the ranger. It is better for the ranger to die than lure the whole horde back to your base.
If you have just started a new game and are using a ranger to reveal the game map then it might be best to just start a new game. Losing a ranger this early in the game might make it very hard for you to progress any further.