leprae is surrounded by the waxy cell envelope coating characteristic of the genus Mycobacterium. Genetically, M. leprae and M. lepromatosis lack the genes that are necessary for independent growth. M. leprae and M. lepromatosis are obligate intracellular pathogens, and cannot be grown (cultured) in the laboratory. The inability to culture M. leprae and M. lepromatosis has resulted in a difficulty definitively identifying the bacterial organism under a strict interpretation of Koch's postulates. While the causative organisms have to date been impossible to culture in vitro, it has been possible to grow them in animals such as mice and armadillos. Naturally occurring infection has been reported in nonhuman primates (including the African chimpanzee, the sooty mangabey, and the cynomolgus macaque), armadillos, and red squirrels. Multilocus sequence typing of the armadillo M. leprae strains suggests that they were of human origin for at most a few hundred years. Thus, it is suspected that armadillos first acquired the organism incidentally from early American explorers. This incidental transmission was sustained in the armadillo population, and it may be transmitted back to humans, making leprosy a zoonotic disease (spread between humans and animals). Red squirrels (Sciurus vulgaris), a threatened species in Great Britain, were found to carry leprosy in November 2016. It has been suggested that the trade in red squirrel fur, highly prized in the medieval period and intensively traded, may have been responsible for the leprosy epidemic in medieval Europe. A pre-Norman era skull excavated in Hoxne, Suffolk, in 2017 was found to carry DNA from a strain of Mycobacterium leprae, which closely matched the strain carried by modern red squirrels on Brownsea Island, UK. Risk factors The greatest risk factor for developing leprosy is contact with another person infected by leprosy. People who are exposed to a person who has leprosy are 5–8 times more likely to develop leprosy than members of the general population. Leprosy also occurs more commonly among those living in poverty. Not all people who are infected with M. leprae develop symptoms. Conditions that reduce immune function, such as malnutrition, other illnesses, or genetic mutations, may increase the risk of developing leprosy. Infection with HIV does not appear to increase the risk of developing leprosy. Certain genetic factors in the person exposed have been associated with developing lepromatous or tuberculoid leprosy. Transmission Transmission of leprosy occurs during close contact with those who are infected. Transmission of leprosy is