the skin multiple times in one spot—usually
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on the upper arm. This intentionally caused a small, localized infection—not with smallpox itself, but with a much milder, related virus called *vaccinia*—which prompted the immune system to prepare to fight the actual disease. Over the next 14–21 days, the area would blister and scab over before healing, leaving behind the small, round, slightly indented scar that many people recognize. Why do so few people have this scar today?
In the United States, routine smallpox vaccination ended in 1972. By
