When you are at a NPC village you may, at some stage, come across a zombie that looks like a villager. They look like normal zombies, but with a green villager head. It is actually possible to cure a zombie villager and turn them back into a normal villager again.
There is a trophy/achievement for performing this which is usually what triggers people to want to do this. The process is fairly simple. You can get a detailed guide on how to cure a zombie villager on the PS4 trophy guide.
The video guide below will show you what you need to do to obtain the trophy and achievement for curing a zombie villager.