People with weakened immune systems should not be exposed to smallpox, they'll surely die, they should also not receive the regular unattenuated viral vaccine because it can be dangerous and can kill those who have no immune systems. Only people who have an immune system should receive the vaccinia virus, it could be very dangerous for AIDS patients, just like chickenpox is a very minor disease for the typical child, but can kill kids on chemotherapy or others with weak immune systems.