I'm trying to find a Jared/Jensen fic I read quite a while ago. I don't remember the name or author but I do rememeber it has Jensen alone initially. Jared then stumbles into his shelter and they hole up together until one day, Jared comes back from a supply run and Jensen finds out Jared got bitten.

Jared eventually turns into a zombo and Jensen is heartbroken and eventually ties Jared up on a mattress to restrain him before having sex with him (can't rememeber if it was once or multiple times). I think the end had Jensen realizing Jared seemed like he started understanding him and that Jared might somehow turning back into a human again.

FOUND by onlythefireborn in comment below! Riyku's "Evolution"