Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This Christmas Day comic takes the classic pickup line format -- "You must be [something], because [romantic compliment]" -- and gives it a grotesque medical twist. A nerdy-looking man at a bar tells a woman, "You must be stage 3 syphilis, 'cause I can't get you out of my head." The caption reads, "Pickup lines are very different in the medical community."

The humor works on two levels. First, it parodies the cheesy pickup line structure by replacing the usual flattering comparison with a horrifying sexually transmitted disease. Second, it is medically accurate in a dark way: tertiary (stage 3) syphilis is known for causing neurosyphilis, which literally affects the brain. So "I can't get you out of my head" is both a romantic cliche and a clinical description of the disease invading the central nervous system. The joke is characteristic of SMBC's fondness for finding comedy at the intersection of romance and science.