A nasty bug going around

Every night, Frank would go down to the off licence, get a six pack, bring it home, and drink it while he watched TV. One night, as he finished his last beer, the doorbell rang. He stumbled to the door and found a six-foot cockroach standing there. The bug grabbed him by the collar and threw him across the room, then left.
The next night, after he finished his 4th beer, the doorbell rang. He walked slowly to the door and found the same six-foot cockroach standing there. The big bug punched him in the stomach, then left.
The next night, after he finished his 1st beer, the doorbell rang again. The same six-foot cockroach was standing there. This time he was kneed in the groin and hit behind the ear as he doubled over in pain. Then the big bug left.
The fourth night Frank didn't drink at all. The doorbell rang. The cockroach was standing there. The bug beat the shit out of Frank and left him in a heap on the living room floor.
The following day, Frank went to see his doctor. He explained events of the preceding four nights. "What can I do?" he pleaded. "Not much" the doctor replied. "There's just a nasty bug going around."