1. Whos is the boy in the attic? 2. What about the parapalegic old Chinese man? The chinese lady raises quite the fuss over her "sick" old father many times, but we never see him. They discuss that he is somewhere upstairs, but that's it.
Right When father Alberta took the possessed medeiros girl to the attick in the building, he wanted to find a cure so her took several experiments on near by children, rats etc That's why the bit came from the attick because when the possessed girl escaped, father Alberta locked himself in the attick where he had put several children he had experimented on
In Rec 2, we realise the little girl was possessed all along.
So it seems the 'virus' develops more slowly in children... And maybe the elderly too. Perhaps the old Japanese man was also possessed all along. This would explain why the whole family eventually gets infected.