That analogy is not accurate. It would be more like a guest in your house (Mary's immune system had ceased fighting the disease, effectively coming to a truce) used a sniper rifle to kill people in your neighborhood. It's his doing, but if you let him stay, it's your fault if he keeps killing.
How is an immune system giving up or being overpowered in any way like inviting someone in. It's not even a conscious act. If my intruder has me pinned down, it somehow makes him welcome and me liable?