system
Given the following post, respond as if you're commenting on Reddit:
human
[Fallout 3] Why would the Lone Wanderer infect Project Purity with the modified FEV?

The only people that wouldn't be killed by it are vault dwellers and the Enclave, and the Lone Wanderer knows they were born in the wasteland and would be killed by it. So why would the Lone Wanderer destroy the Capital Wasteland for the sake of the Enclave?
gpt
Eden brings this up when you ask him about it

> Anyone or anything that has been affected by mutation will be eliminated. You will likely be immune, thanks to your upbringing in the vault. Likewise, the good people of the Enclave will be unaffected as well. 

Presumably it’s not just being in the wasteland when you’re born that would cause you to be affected. It’s remaining there and being exposed to the radiation long enough for it to mutate you. The Lone Wanderer was born there, but pretty much immediately brought to the vault and shielded from radiation after that. That’s not necessarily a guarantee they’ll be immune, or that Eden was being honest, but spending the majority of their life in a vault might’ve been enough. As it turns out they’re not immune, but they did have reason to suspect they might’ve been.