Bed bugs can start in one room of your house and before you know they are in every room. How did that happen?
Bed bugs are hitchhikers and can walk from room to room. One person can bring them in through luggage, clothes, backpack, or a friend.
If an overnight guest brings them in then the bugs are are fine to stay in that one room until the person leaves. Once that person leaves or does not sleep there the bugs will get hungry. If there not a person to feed on, they will literally walk right out of the room to another.
People can also lay on a spot and pick them up and take then other rooms.
It is important to have a professional treat as soon as you think you have them. Waiting can spread the bugs and make getting rid of them take longer.