Leprosy, also called Hansen’s disease, is a chronic bacterial infection that primarily affects the skin and nerves. It is caused by slow-growing bacteria called Mycobacterium leprae (and a related species, Mycobacterium lepromatosis) and remains one of the oldest known infectious diseases. Despite centuries of stigma, leprosy is curable with antibiotics, and the vast majority of people exposed to the bacteria never develop the disease.
What Causes Leprosy
The bacteria behind leprosy reproduce extraordinarily slowly compared to most infectious organisms. This slow growth rate is why the disease has one of the longest incubation periods of any infection: after exposure, it can take anywhere from a few years to as long as 20 years before symptoms appear. Most people’s immune systems can fight off the bacteria entirely, which is why leprosy has never been highly contagious even in areas where it circulates.
Transmission is thought to occur mainly through respiratory droplets during prolonged, close contact with an untreated person. Casual or brief encounters carry very little risk. In the United States, about a third of patients have no history of foreign travel or known contact with someone who had the disease, pointing to local sources of infection. Nine-banded armadillos in the southeastern U.S. are naturally infected with M. leprae, and genetic analysis has linked armadillo strains to roughly 42% of human cases in that region. Hunting, handling, or eating armadillos appears to be a genuine, if uncommon, route of exposure.
Signs and Symptoms
Because the bacteria target the skin and peripheral nerves, most symptoms fall into two categories.
Skin changes are often the first noticeable sign. These include patches of skin that are lighter or more discolored than surrounding areas, firm rounded bumps under the skin, thickened or unusually dry skin, painless ulcers on the soles of the feet, and painless swelling or lumps on the face or earlobes. Some people lose eyebrows or eyelashes.
Nerve damage can develop alongside or after skin symptoms. Affected areas of skin may become numb, and muscles in the hands and feet can weaken or become paralyzed over time. Nerves near the elbows, knees, and sides of the neck sometimes become visibly enlarged and can be felt through the skin. Eye involvement is also possible and, if untreated, can lead to blindness.
Because these symptoms develop gradually over months or years, many people don’t seek medical attention until the disease has already progressed.
How Leprosy Is Classified
The World Health Organization classifies leprosy by how many skin lesions a person has, which guides treatment decisions. A single lesion is its own category. Two to five lesions is called paucibacillary leprosy, meaning fewer bacteria are present. More than five lesions is called multibacillary leprosy, indicating a higher bacterial load and typically a more extensive disease course. Multibacillary cases require a longer course of treatment and carry a greater risk of complications.
How It Is Diagnosed
Diagnosis starts with a physical exam. A doctor looks for the characteristic lighter skin patches, tests for numbness by touching the patches, and feels for enlarged nerves. A skin biopsy, usually taken from the edge of an active patch, confirms the diagnosis. Lab technicians use a special staining technique to identify the bacteria under a microscope. If acid-fast bacilli show up in the sample, the diagnosis is confirmed.
For multibacillary cases, skin smears from the earlobes, elbows, and knees can reveal the bacteria in large numbers. In some samples, the bacteria are found inside immune cells called foam cells, which have swallowed the organisms but cannot destroy them. When clinical findings are unclear, PCR testing can detect the bacteria’s genetic material, though this is only available at specialized laboratories.
What Happens Without Treatment
Left untreated, leprosy causes progressive, irreversible nerve damage. The numbness itself becomes dangerous because people can’t feel injuries, burns, or infections on affected hands and feet. Repeated unnoticed trauma leads to chronic wounds, secondary infections, and gradual tissue loss. Over years, the bones of the fingers and toes can shorten through a process called resorption, where the body slowly absorbs damaged bone.
One of the most recognizable complications is claw hand, where nerve damage to the ulnar and median nerves causes the muscles of the hand to waste away. The fingers curl inward, and the palm flattens as the fleshy pads at the base of the thumb and pinky atrophy. A case report from Colombia illustrates the extreme end of this spectrum: a man who had hunted armadillos as a child developed glove-like numbness and severe claw deformities in both hands over 13 years, along with finger shortening from bone resorption and widespread skin changes. By the time he sought care, much of the damage was permanent.
Facial nerve involvement can prevent the eyelids from closing fully, a condition that dries out the cornea and can cause blindness. Foot drop, where the foot drags during walking, results from nerve damage in the lower leg.
Treatment and Recovery
Leprosy is fully curable with a combination of antibiotics, typically taken for six months to a year depending on the classification. Paucibacillary cases require a shorter course, while multibacillary cases need 12 months. Treatment kills the bacteria and stops the disease from progressing, but it cannot reverse nerve damage that has already occurred. This is why early diagnosis matters so much.
During and after treatment, some people experience inflammatory flare-ups called reactions, where the immune system mounts a sudden response against bacterial remnants in the body. These reactions can cause new nerve pain, skin inflammation, and swelling, and they require additional treatment to prevent further nerve damage. Reactions can occur even after the bacteria have been eliminated.
Prevention
There is no vaccine designed specifically for leprosy, but the BCG vaccine (originally developed for tuberculosis) offers partial protection. A meta-analysis published in The Lancet Infectious Diseases found that BCG provided roughly 26% protection in controlled trials, while real-world observational studies suggested higher effectiveness around 61%. A second dose of BCG appeared to boost protection beyond a single dose. In countries with high leprosy rates, BCG vaccination in infancy likely prevents some cases, even if it doesn’t eliminate risk entirely.
For people living in close contact with someone diagnosed with leprosy, preventive antibiotic treatment can reduce the chance of developing the disease. Avoiding direct contact with wild armadillos, particularly in the southeastern United States, also lowers risk for people in those regions.
Global Impact Today
Leprosy is far less common than it once was, but it has not disappeared. In 2024, 188 countries reported a combined 172,717 new cases to the WHO. About 40% of those cases were in women, and 5.4% were in children, indicating ongoing transmission in affected communities. Over 9,000 new cases already showed visible disability at the time of diagnosis, a sign that many people are still being detected too late. The highest rates of new cases are concentrated in sub-Saharan Africa and Southeast Asia, though cases occur on every inhabited continent.