 Leprosy is a chronic infectious disease caused by mycobacterium lepre, which has been known for centuries. In 2011, it was estimated that there were 1.54 cases of leprosy per 10,000 people living in Brazil. Transmission of the disease occurs when untreated multibasillary patients come into close contact with susceptible individuals. The disease can also be transmitted by inhaling bacilli from the upper respiratory tract. The nasal mucosa is the primary entry point for M. lepre. Understanding the structure and biology of M. lepre, as well as the genetic factors involved in the host's immune response, has led to greater insight into the pathophysiology of the disease. This article was authored by Joel Carlos Lastoria and Marilda Aparecida-Mailanes-Morgado del Bru.