Question about movie Quarantine? (If you have seen it)?

You know at the end where they go into the attic to find a way out and
that guy has a bunch of rats.. That creepy creature eating the people
in the attic, was that the old man who they hadn't seen for months and
he was like some alien creature thing?? Or was that one of his
creations?? Cause I think it was the old man who they hadn't seen for
months but my parents think it was one of his creations or just some
monster thing... Do you know what it was?