Leprosy or Hansen's Disease, is a chronic infectious disease caused by the bacterium Mycobacterium leprae. Leprosy can be treated effectively with several drugs, but if left untreated, the disease can result in severe disfigurement, especially of the feet, hands, and face. It is rarely fatal.

Leprosy has long been one of the most feared diseases worldwide. The stigma attached to leprosy has often caused those who contracted the disease to be shunned by family, friends, and society. For example, in Europe during the Middle Ages (5th century to 15th century), people with leprosy were declared dead and were banished after witnessing their own funeral and symbolic burial. Confined to a leprosarium or forced to wander and beg to survive, these outcasts were required to warn others of their presence with a bell or clapper.

Although leprosy was once widespread throughout the world, today it is primarily a tropical disease. More than 95 percent of leprosy cases occur in just 11 nations, and India and Brazil have the largest numbers of cases. Due to effective treatment and leprosy eradication programs, the prevalence of the disease has declined dramatically in recent years. According to the World Health Organization (WHO), in 1985 there were 5.4 million registered cases of leprosy and an estimated 10 to 12 million total cases worldwide. By 2000, there were only 680,000 registered cases and an estimated 1.6 million total cases of leprosy worldwide. Fewer than 7,000 registered cases of leprosy currently exist in the United States. Each year some 300 new cases of leprosy are identified in the United States. The vast majority of these patients are immigrants who acquired the disease in their home countries.

Leprosy has two main forms, known as tuberculoid and lepromatous disease. In tuberculoid leprosy, the skin lesions are few and small, with only a few bacteria present in each. In lepromatous leprosy, the more severe form of the disease, the lesions may be much more widespread and contain many leprosy bacteria. As lepromatous leprosy progresses, hard nodules and folds of skin may form on the face and the nose may collapse, giving a person a characteristic lionlike appearance.

The symptoms of leprosy may be caused by proliferation of the bacteria in lepromatous leprosy or by the body’s immune response to the bacteria in tuberculoid leprosy. In both forms of leprosy, there is usually some degree of irreversible nerve damage resulting from either of these two processes. Because of the lack of sensation in affected areas of the skin, people with leprosy often do not notice burns and injuries to their fingers and toes and fail to treat them. These injuries can then become infected with other types of bacteria that cause tissue damage. Gradually, damaged tissue and bone are resorbed by the body, causing the digits to become shorter. However, leprosy does not, as myth would have it, cause parts of the body to fall off. Damage to nerves in the hands and feet may also cause the fingers and toes to become stiff and curl inward, and some patients become unable to walk. Both forms of the disease may also lead to blindness.

Throughout much of history, people believed leprosy to be a highly contagious disease, fearing that merely touching an infected person could spread the disease. Today scientists know that leprosy is not easily transmitted, but they are still not sure how it is spread from person to person. Nasal droplets released when a person with untreated lepromatous disease sneezes may contain large numbers of leprosy bacteria. Conceivably, these released bacteria could infect a new person who inhales the droplets, or the bacteria could invade through a cut or abrasion in the person’s skin. Scientists suspect that these processes may be the primary means of spreading the disease, but do not know for certain. In any event, the well-known case of Father Damien, a Belgian missionary who contracted the disease while caring for leprosy patients on the Hawaiian island of Molokai during the late 1800s, appears to be the exception rather than the rule. Only very rarely do health-care workers who care for patients with leprosy develop the disease themselves.

Scientists estimate that less than 5 percent of people who are infected with Mycobacterium leprae actually develop leprosy. In most cases, the immune system easily fights off the infection. Scientists do not know why a few individuals develop the disease while most people are naturally immune. Those who develop leprosy do not have weak immune systems in general—for example, they do not have an increased susceptibility to other infections or to cancer.

Even in people who do develop symptoms, the leprosy bacterium appears to be a relatively weak disease-causing agent. Though the complications of leprosy can be devastating, the disease is rarely fatal. In fact, the symptoms are surprisingly mild given the large numbers of bacteria that are often found in the skin lesions. The leprosy bacterium multiplies very slowly—once every 2 weeks, compared with once every 24 hours for the closely related bacterium that causes tuberculosis, and once every 20 minutes for certain other bacteria. This slow rate of reproduction also means that the disease develops very slowly—2 to 10 or more years can go by before an infected person develops symptoms.