Leprosy is not hereditary. It is a chronic infectious disease caused by bacteria called Mycobacterium leprae, spread through respiratory droplets during prolonged, close contact with an untreated person. It is not passed down through genes the way conditions like sickle cell disease or cystic fibrosis are. However, your genes do play a real role in whether you’re susceptible to the infection, which helps explain why leprosy sometimes seems to run in families.
Why Leprosy Seems to Run in Families
The reason leprosy was long mistaken for a hereditary condition is straightforward: it clusters in households. When multiple family members develop the disease, the pattern looks genetic at first glance. But the clustering has a simpler explanation. Leprosy requires months of close, repeated contact with someone who is untreated and actively shedding bacteria through coughs and sneezes. People who share a home with an untreated case are far more likely to have that kind of sustained exposure than anyone else.
Research on household contacts has found that roughly 13 to 18 percent of the variation in who develops leprosy among contacts can be attributed to characteristics of the person they live with, including the severity of that person’s infection. In other words, shared environment and prolonged proximity, not shared DNA, drive the family pattern.
Genetics Affect Susceptibility, Not Transmission
That said, genetics are not irrelevant. Around 95% of people who encounter M. leprae will never develop the disease because their immune system clears the bacteria on its own. What separates the other 5% is partly a matter of immune system genetics.
A large genome-wide study published in the New England Journal of Medicine identified variants in several genes that influence susceptibility. These include genes involved in the body’s innate immune signaling, the first-response system that detects and fights off unfamiliar bacteria. Variants in a gene region called HLA-DR, which helps immune cells recognize and respond to foreign proteins, showed a particularly strong association. People carrying certain HLA-DR variants had roughly twice the odds of developing leprosy compared to those without them. Other genes in the same immune pathway, including NOD2 and RIPK2, also showed significant links.
What this means in practical terms: you can inherit a genetic profile that makes your immune system less effective at fighting off this specific bacterium. But inheriting that profile does not give you the disease. You still need to be exposed to the bacteria through close contact with an untreated person. The genetic susceptibility and the environmental exposure are both required.
Can a Mother Pass Leprosy to Her Baby?
Leprosy is not transmitted during pregnancy in the way genetic conditions are. The CDC states clearly that it is not passed to the fetus during pregnancy. However, the picture is slightly more complicated than a simple “no.”
Studies have found that the bacteria or its components can, in some cases, cross the placenta. Researchers have detected M. leprae in umbilical cords and cord blood of babies born to mothers with active leprosy, and antibodies against the bacteria have been found in cord serum. This suggests that some fetal exposure can occur. But this type of vertical transmission is considered uncommon and does not mean the baby will develop the disease. It depends on a combination of factors: whether live bacteria are present in the mother’s blood, how far along the pregnancy is, and how the baby’s developing immune system responds.
Babies born to mothers with leprosy do face some broader health risks, including lower birth weight and higher rates of infection generally, so close follow-up care is recommended for early detection.
How Leprosy Actually Spreads
The bacterium spreads through airborne droplets from the nose and mouth of someone with untreated leprosy. This requires prolonged, repeated exposure over many months. You cannot catch it from shaking hands, hugging, sharing a meal, or sitting next to someone. You also cannot catch it through sexual contact.
Once a person with leprosy starts treatment, they stop being contagious. The bacteria are quickly rendered non-transmissible by the standard antibiotic regimen. This is one reason early detection matters: treatment protects both the patient and the people around them.
The disease also has an unusually long incubation period. After exposure, M. leprae can take years to produce symptoms, sometimes as long as 20 years. This long delay is another reason the disease historically appeared hereditary. A child exposed in infancy might not show signs until adulthood, long after anyone connected the dots to the original household exposure.
Leprosy Today
Leprosy remains a global health concern, though far less widespread than it once was. In 2024, 188 countries reported data to the WHO, documenting 172,717 new cases worldwide. About 40% of new cases were among women, and 5.4% were among children. The disease is curable with a combination of antibiotics, and early treatment prevents the nerve damage and disability historically associated with the condition.
The persistence of new cases, including among children, signals that transmission continues in some communities. But the old belief that leprosy passes inevitably from parent to child is wrong. It is a bacterial infection shaped partly by genetic susceptibility, spread through the air during close contact, and stopped by treatment.