Leprosy spreads through respiratory droplets, but it requires prolonged, close contact with an untreated person over many months to catch it. It is one of the least contagious infectious diseases, and the vast majority of people exposed to the bacteria never develop symptoms. Despite its reputation as a highly feared illness, your chances of contracting leprosy from brief or casual encounters are essentially zero.
How Leprosy Actually Spreads
The bacteria that cause leprosy travel in tiny droplets released when an infected person coughs or sneezes. For transmission to occur, a healthy person needs to breathe in those droplets repeatedly over a long stretch of time. This isn’t a single-exposure infection. It requires months of close, sustained contact with someone who has active, untreated disease.
You cannot get leprosy from shaking hands, sitting next to someone, or having a conversation. It does not spread through sexual contact, and a pregnant person with leprosy does not pass it to the fetus. Touching the skin lesions of a person with leprosy is not considered a meaningful route of transmission either. The disease overwhelmingly spreads through the respiratory route, not through skin-to-skin contact.
Armadillos Are a Source in the US
In the United States, leprosy also has an animal reservoir: nine-banded armadillos. These animals carry the same bacteria and have been linked to human cases in Texas, Louisiana, Alabama, Georgia, and Florida. People who hunt, handle, or eat armadillos in these states face a small but real risk of infection. Outside of direct contact with armadillos, leprosy cases in the US are rare, with most infections occurring in people who spent time in countries where the disease is more common.
Why Most People Can’t Catch It
Even with prolonged exposure, the overwhelming majority of people never develop leprosy. Roughly 95% of the human population has a natural immune response strong enough to fight off the bacteria. This is why household contacts of leprosy patients usually remain healthy, and why the disease has never spread explosively through populations the way flu or tuberculosis can.
The small number of people who are susceptible likely have a genetic predisposition that makes their immune system less effective at clearing this particular bacterium. Researchers have identified specific immune pathways involved in resistance, but there’s no simple test to determine whether you’re in the susceptible minority.
A Very Slow Infection
The bacterium that causes leprosy multiplies extraordinarily slowly compared to most other infectious agents. The incubation period averages about five years, but symptoms can take as long as 20 years to appear. In some cases, signs show up within a year of exposure. This long delay between infection and symptoms makes it difficult to pinpoint exactly when or where someone was exposed, and it means many people don’t realize they’re carrying the bacteria for years.
Early symptoms typically include pale or reddish patches on the skin that have reduced sensation. Because the bacteria target nerve cells, numbness or tingling in the hands or feet is often one of the first signs. Left untreated, the infection gradually damages peripheral nerves, which can lead to muscle weakness, loss of feeling in the extremities, and the disfigurement historically associated with the disease.
Two Bacteria, One Disease
Most cases of leprosy are caused by a bacterium identified in the late 1800s. In 2008, scientists discovered a second, closely related species responsible for a particularly severe form of the disease. The two species diverged roughly 14 million years ago and differ by about 9% in their genetic code. Both invade nerve cells using the same attachment mechanisms, which is why they produce similar symptoms.
The second species is primarily associated with a form of leprosy that causes widespread skin involvement and, in severe cases, large areas of dead or ulcerated skin, particularly on the legs. This form can lead to dangerous secondary infections. It remains far less common than the standard type, and most clinicians outside of specialized centers may never encounter it.
Treatment Stops Spread Immediately
Once a person begins antibiotic therapy, they are considered non-infectious after the very first dose. The treatment involves a combination of antibiotics taken over six months to a year for milder cases, or up to two years for more advanced disease. This multi-drug approach is highly effective and cures the infection in nearly all patients.
For people who have been in close contact with a newly diagnosed case, the World Health Organization recommends screening and, in some programs, a single preventive dose of an antibiotic to reduce the risk of developing the disease. This approach has shown promise in reducing new cases among household contacts.
How Common Leprosy Is Today
Globally, about 173,000 new cases of leprosy were reported in 2024 across 188 countries. Around 40% of those cases were in women, and about 5% were in children. The disease is concentrated in tropical and subtropical regions, with India, Brazil, and Indonesia accounting for the majority of cases worldwide. In the US, roughly 150 to 200 new cases are diagnosed each year, most of them in people who emigrated from or traveled to areas where leprosy is endemic.
The number of global cases has dropped dramatically over the past several decades thanks to widespread access to effective treatment, but leprosy has not been eliminated. Delayed diagnosis remains a significant challenge, partly because the long incubation period means people may not seek care until nerve damage has already begun.