Atomic Heart

While in Vavilov you'll need to collect 4 canisters to bring a tree back to life. Earliest opportunity to unlock this achievement is during the mission "March of the Flowers." Which is to collect the 3rd canister from the Cryo Labs. Near the start of the mission you'll enter a large room filled with green plants, if you scan the room you'll see "Mothers" sat amongst the plants. These "Mothers" will spawn the Sprouts so don't kill them just yet!

Once you've completed the battery puzzle and powered up the room the Mothers will wake up and start launching Sprouts into the air. When a Sprout comes into contact with a dead body it'll transform it into a Mutant. The key here is to stand near a dead body and wait for a Sprout to enter it. Soon as the Sprout enters the dead body quickly hit it with a melee weapon to kill the host. If killed before turning the achievement should pop.

You'll get several other opportunities to get this achievement while inside the Theatre and Pavlov.

Missable:

Best obtained inside of Vavilov during the Cryo section with all of the planters. You’ll face an onslaught of Sprouts. There are several corpses around. Wait by one of the corpses and get a Sprout to infect that corpse by luring it towards it. When the body starts to wiggle/the spout enters destroy the body before it gets up as a full fledged mutant.

Edit: There are plenty of opportunities to get this elsewhere though!

During the March of Flowers mission where you are waiting for the cannister to fill there will be sprouts that spawn. Wait for one to leap at/jump in one of the dead bodies on the floor and kill the sprout before it turns it into a mutant.

This will be the spot to unlock this as there are only a handful of fights in the game where you face sprouts.

There are many opportunities to get this during the course of the game but you can do it for the first time in the pesticide lab, I chose the area just before the fight with the giant mutant since there are no enemies around and you can save in the room where you blow up the tank just in case.

During the March of Flowers mission where you are waiting for the cannister to fill there will be sprouts that spawn. Wait for one to leap at/jump in one of the dead bodies on the floor and kill the sprout before it turns it into a mutant.

This will be the spot to unlock this as there are only a handful of fights in the game where you face sprouts.