If you’ve ever noticed a round or slightly indented scar on the upper left arm of an older relative—or even on your own arm—you’re looking at a quiet remnant of one of the deadliest diseases in human history. That small mark is most often the result of a smallpox vaccination, a medical intervention that helped wipe out a virus that once terrorized the world.
Smallpox was caused by the variola virus, a highly contagious pathogen that spread easily from person to person through respiratory droplets or direct contact. The disease typically began with a high fever, fatigue, and body aches, before erupting into a painful, pus-filled rash that covered much of the body. By the early 20th century, smallpox outbreaks were still widespread and devastating, killing roughly three out of every ten people who became infected. Those who survived were often left with permanent scars, particularly on the face, serving as lifelong reminders of the illness.
In the face of such destruction, scientists searched for a way to prevent the disease rather than merely treat it. Their breakthrough came in the form of a vaccine developed using vaccinia, a live virus closely related to variola. While vaccinia does not cause smallpox, it is similar enough to train the immune system to recognize and fight off the real thing. When administered, the vaccine prompted the body to produce antibodies that could rapidly neutralize the smallpox virus if exposure ever occurred.
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