It can spread through 3 methods:
Bluetooth
Hard drive injection
Bites.
Blue tooth was how it got around at first,then the Animatronics got smart and removed thier Bluetooth chips. Then it moved to Hard drive injection. The infected forcibly took the hard drives from asleep Animatronics and injected the virus into it. They got smart  again and slept in shifts. Then it moved to bites. An infected Animatronic could bite a noninfected and spread it to them. It could infect a person at this stage,but it never did and never will.