This was two or three years ago, so if anyone can help out with this it will be greatly appreciated.
EDIT: Found by demonspot - it's "Resident Evil" and can be read here.
Title: Four Seasons
Rating: R, only for swearing at the moment
Disclaimer: Not mine, all Joss’s.
Summary: Looking back, Spike had never worked out just how he and Harris got to where they did that summer; he couldn’t quite work out exactly where that was either.
Beta thanks to laazikaat
Other chaps here
Nothing the Same, Book 3
Rating: PG13 - NC-17 Individual chapters will carry specific warnings.
Feedback & concrit: yes, please
Disclaimer: don't own them, never will, just playing with them
Spoilers: Primarily season 4, but anything from Season 1 on.
Summary: sequel to Nothing the Same & Nothing the Same, Book 2
Previous parts here