The eradication of smallpox is one of humanity's great achievements - but the battle against the virus was fought by the most unlikely of alliances. How did the breakthrough happen - and can we guarantee that the world is still safe from smallpox?

This episode owes a debt to Stephen Coss’s book The Fever of 1721, Ibram X. Kendi’s book Stamped From the Beginning, and to an article about Dark Winter written by Tara O’Toole, Michael Mair and Tomas Inglesby.

For a full list of our sources please see the shownotes at http://timharford.com/

Tim's latest books 'Fifty Inventions That Shaped The Modern Economy' and 'The Next Fifty Things That Made The Modern Economy' are available now.