This is a Twitter essay on the history of leprosy, to commemorate World Leprosy Day, 29 January 2023. I’ve chosen for my theme this year the “bookend” discoveries of the two known species of bacteria that cause leprosy: that is, the discovery of Mycobacterium leprae by Armauer Hansen 150 years ago, and the discovery of Mycobacterium lepromatosis by Xiang-Yang Han 15 years ago. The transition from whole-cell bacteriology to molecular genomics has been transformational in our ability to piece together the global history of the disease. The present document brings together all the original tweets, and supplements them with a bibliography.