What Is the Cause of Leprosy?

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  • Written By: Jessica Ellis
  • Edited By: Bronwyn Harris
  • Last Modified Date: 25 October 2019
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The cause of leprosy, also known as Hansen's disease, eluded the medical practice for centuries. While some thought the disease a curse or a punishment from God, the true cause of leprosy resides in two types of bacterial infection. Since this discovery was made in the 20th century, antibiotics have been developed that can treat and cure the disease in the early stages.

Leprosy has existed in the human population for thousands of years. Written accounts of the disease date back to at least the 6th century BCE, and come from various cultures around the world. The distinct symptoms of the disease, which include disfiguration, weakness, and destruction of nervous and muscular tissue, have long made victims of the illness a target for isolation and abuse. Although the disease is better understood in the modern world, it remains a serious condition that requires prompt medical treatment.

One of the major difficulties in isolating the cause of leprosy is the extremely long incubation period of the disease. While some patients may manifest symptoms within weeks of exposure, others may not display any signs of the disease for more than a decade. Historically, this inconsistency in symptom development made it almost impossible for medical scientists to track down the source of the disease and understand how it spreads; not until the development of microscopic studies and modern antibiotic therapy could the true cause of leprosy be discovered.


The cause of leprosy can be one of two bacteria, Mycobacterium leprae, and Mycobacterium lepromatosis. Both bacteria come from the same family as tuberculosis, and some cases of leprosy may be classified as tuberculin infections, depending on the symptoms. Infection is believed to be transmitted through the inhalation of respiratory secretions, such as mucous or saliva. In some cases, the bacteria can also be transmitted from animals to humans, most notably from armadillos.

While the bacteria is the only known cause of leprosy, several risk factors may increase the chance of infection. High instances of the disease are linked to extreme levels of poverty and the resulting lack of adequate sanitation or clean drinking water. It also tends to be disproportionately found in tropical or sub-tropical climates, though it can occur elsewhere. The most important risk factor for leprosy may be the presence of certain genes, which are known to be susceptible to the disease. According to some experts, less than 10% of the world's population may possess these genes, meaning that most people will be fully immune to the disease.


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