Visible crew/equipment: When the priest arrives and when he later walks to the house, lighting screens are reflected on his glasses.
Visible crew/equipment: In the scene where Bill Pullman is in his office room, in the mansion, the 3 ghosts lift him in the air, you can see the wires lifting the chair.
Chosen answer: It was most likely Pneumonia, which was very deadly before the discovery of antibiotics.
Donald Jenkins