It’s a disease that has horrified society since Biblical times, with civilizations both ancient and current casting out those who became incurably infected. And it remains with us thousands of years later, even having its own day on the calendar: Jan. 29 is World Leprosy Day. What is leprosy and why can we still get it?

Modern leprosy, formally known as Hansen’s disease, is an infection caused by certain bacteria. According to the Centers for Disease Control and Prevention, the bacteria grow slowly, so it could be a decade before you even know you’ve contracted the illness. When the symptoms do appear, they may include skin lesions and growths, hardened skin, pain, numbness, enlarged nerves and muscle weakness. Some people with leprosy experience “eye problems that may lead to blindness.”

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When left untreated, leprosy “can cause progressive and permanent damage to the skin, nerves, limbs and eyes,” the World Health Organization warns.

You can still become infected if you frequently come into contact with an untreated person, through droplets from their nose and mouth, although the potentially disabling disease can now be cured. The WHO says leprosy was “eliminated” as a public health issue by 2000 — fewer than one in 10,000 people around the globe have it — with the help of an effective drug treatment developed in the second half of the 20th century.

“The disease was once feared as a highly contagious and devastating disease,” the CDC says. “Now, however, the disease is very rare and easily treated. Early diagnosis and treatment usually prevent disability related to the disease.”

The U.S. National Library of Medicine adds that leprosy is not especially contagious. “Most people who come in contact with the bacteria don't develop the disease. This is because their immune system is able to fight off the bacteria.”

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That doesn’t mean leprosy is anything to scoff at — it has infected humans over many eras and shows no signs of leaving. While some argue that the leprosy described in the Bible is not the same disease that infects us today, it’s at least the same as the leprosy of the Middle Ages, according to scientists who examined the remains of lepers who died between the 10th and 14th centuries. And new research suggests leprosy picked up during that time period because of how people were traveling.

For more information, visit the American Leprosy Missions.

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