Help me understand something here....

So there was a hallway, a red door, then a small entryway with another door that went outside.
The dog was in the small entryway room between the doors.

When Travis was in the hallway was the red door unlocked?
And how did the dog get inside to be in the entryway between the doors?
Paul and his wife suspected Andrew somehow let the dog in but Will's wife said he could not reach the locks.
This is how Andrew would have gotten infected which he then passed on to Travis.
Right?
So we just figure Andrew let the dog in and the mom was lying about him not reaching the locks?
Otherwise how else would the dog get in bringing the infection?