I heard that to get this trait you need to either kill a huge number of zombies or survive for a very long time. But I have not found any specific numbers or data anywhere, and therefore I have no idea how to get it. For example, on my current survival, I have lived for 25 days and killed more than 10,000 zombies, but I still have not received this trait.

Can someone explain to me in detail how the mechanism for obtaining "Desensitized" trait works?