Astonishing story in The Atlantic, about the smallpox vaccine:

At the heart of history’s most successful eradication campaign is a mystery. The smallpox vaccine—now also being deployed against monkeypox—contains a live virus that confers immunity against multiple poxviruses. But it is not smallpox or a weakened version thereof. Nor is it monkeypox. Nor is it cowpox, as suggested by the vaccine’s famous origin story involving pus taken from an infected milkmaid to immunize an 8-year-old boy.

The vaccine predates systematic, controlled manufacture, so several or many versions were made from various sources. And they were transferred by sending infected children around the world! I do seem to recall hearing that last part before, but not the fact that its true origin is shrouded in mystery.

Well worth a read. Via Kottke