How rare are zombie villagers?

Zombies have a 5% chance of naturally spawning as zombie villagers. This means that roughly every 1 in 20 zombies you find will be a zombie villager, making them "rare", but you should be able to find at least one every night while exploring.

One important thing to note is that in desert biomes, zombies will have an 80% chance of spawning as husks, which CANNOT turn into zombie villagers. This will make finding them in the desert 5 times harder.

If you are hunting for Zombie villagers, I suggest you find a populated village and stay until a zombie siege occurs. On Hard difficulty, villagers who are killed by Zombies have a 100% chance of becoming a zombie villager. This decreases to 50% on Normal, and 0% on Easy.

Notes:

• Each igloo with a basement is guaranteed a zombie villager and a normal villager.
• Zombie villagers also have a 5% chance of spawning as baby form.

For more info check out the wiki pages for this: Zombie#Spawning and Zombie Villager#Spawning

Zombie Villagers spawn when a Zombie kills a Villager.

In Easy there is a 0% chance after a kill

In Normal there is a 50% chance after a kill

In Hard and Hardcore there is a 100% chance after a kill.

They also have a 5% chance to spawn naturally.