Are you saying Order of the Pheonix as the book or as the place? Because I'm pretty sure that the battle took place in the Ministry of Magic.
And by place I meant group. The Order of the Pheonix is a secret society and Number 12 Grimmauld Place was their headquarters. But my point was that a battle taking place inside a group sounds a lot like a viral infection.
Although I guess it could have been a battle within an organization, but that would be weird, seeing as Voldemort never would've joined the Order.