About 95 percent of people are naturally immune to leprosy, and casual contact does not spread it, yet the disease drove societies to banish its victims to places like Molokai in Hawaii. This episode explains the real biology of Mycobacterium leprae and Mycobacterium lepromatosis, bacteria that cannot make their own energy and must live inside host cells. They invade Schwann cells, which insulate peripheral nerves, and reprogram them into a stem cell like state that migrates and spreads the infection. The resulting loss of myelin destroys the ability to feel pain, so unnoticed burns and cuts become infected, and it is those secondary infections that shorten fingers and toes.

Transmission is thought to be by respiratory droplets over months of close contact, with symptoms appearing after an average of five years. Susceptibility is linked to genes including PARK2, also involved in Parkinson’s disease. Researchers grew the bacteria in mouse foot pads and nine banded armadillos, and genetic typing suggests Europeans gave leprosy to American armadillos. In 1915 Alice Ball made chaulmoogra oil injectable, and the multidrug therapy of the 1980s cut cases from about 5.2 million to under 200,000.

• Paucibacillary and multibacillary disease, depending on whether the immune response walls off the bacteria
• Red squirrels on Brownsea Island carry a strain close to one found in a pre Norman skull from Suffolk
• Gregory of Nazianzus called it a holy disease, while Daniel Cornelius Danielssen in Norway thought it hereditary
• Rifampicin, dapsone, and clofazimine make patients non infectious within 72 hours, and the WHO provides them free
• Fear of losing jobs and marriages still leads people to hide early symptoms