Although the mode of transmission of Hansen's disease remains uncertain, most investigators think that M. leprae is usually spread from person to person in respiratory droplets. Studies have shown that leprosy can be transmitted to humans by armadillos. Leprosy is now known to be neither sexually transmitted nor highly infectious after treatment. Approximately 95% of people are naturally immune and sufferers are no longer infectious after as little as 2 weeks of treatment.
The minimum incubation period reported is as short as a few weeks, based on the very occasional occurrence of leprosy among young infants. The maximum incubation period reported is as long as 30 years, or over, as observed among war veterans known to have been exposed for short periods in endemic areas but otherwise living in non-endemic areas. It is generally agreed that the average incubation period is between three and five years.
Leprosy has affected humanity for over 4,000 years, and was well-recognized in the civilizations of ancient China, Egypt, and India. In 1995, the World Health Organization (WHO) estimated that between 2 and 3 million people were permanently disabled because of leprosy at that time. In the past 20 years, 15 million people worldwide have been cured of leprosy. Although the forced quarantine or segregation of patients is unnecessary in places where adequate treatments are available, many leper colonies still remain around the world in countries such as India (where there are still more than 1,000 leper colonies), China,Romania,Egypt, Nepal, Somalia, Liberia, Vietnam, and Japan. Leprosy was once believed to be highly contagious and was treated with mercury — all of which applied to syphilis, which was first described in 1530. It is now thought that many early cases of leprosy could have been syphilis[clarification needed].