This achievement is for having the combo of Polyphagia, Autophagia and Cannibalism evolved using the Necroa virus.
I was having trouble unlocking this when I already had zombies. So it may or may not unlock during that. However, it is really easy to get going directly for it.
Start a game on casual difficulty with the Necroa Virus. Your gene setup doesn't really matter but ATP Boost, Native Biome and Xerophile may speed things up a bit. Start in India.
As soon as you get enough points for each symptom, evolve these:
Now just wait. No need to do anything else since you don't need to win the game or anything to unlock this. It should pop up that you've discovered the combo after a little while.. achievement unlocked. Now feel free to quit out if you wish.