Smallpox once killed millions, and the round scar some older people carry is often from the vaccine that helped eliminate it. The illness, caused by the variola virus, spread easily and produced a severe fever followed by a blistering rash, killing about 30% of those infected and scarring many survivors. Protection came from a vaccine using the related vaccinia virus, which safely trained the immune system to recognize and destroy smallpox. That tiny scar is a lasting mark of one of medicine’s greatest victories.
