Apropos of Y. pestis (plague) genes being found in a 4000 year old skeleton, @Sylvia Knight was wondering out loud why the sudden catastrophic wave of the disease, the Black Death, in the 13th century.

So I did a bit of reading. There was a fringe theory a couple of decades ago that black death was not bubonic plague. One of the reasons some people thought this is from epidemiology: it spread too fast. Specifically, it travelled around Europe at the speed humans did, not at the speed rats did, suggesting rat fleas were not responsible for primary transmission, as many of us were taught at school.

Turns out that, however, human fleas can also transmit the disease, but they're really shit at it and probably not the culprit.

But body lice, they are like, actually quite good at transmitting it. Apparently coming into contact with a plague corpse 3 days post mortem, if that corpse had body lice, is sufficient to infect you with around 10 bites. Really not many.

Body lice don't live on warm human bodies - it's not comfortable for them. They live in our clothes.

Or rather, they used to. Laundry detergent eradicated them.

But laundry detergent is a 20th century invention, and the plague stopped coming to Europe in the 17th century.

Soap was around before though, a long time before.

Very popular in the Middle East towards the end of the 1st millennium.

Which means, of course, in Christian Europe, it became associated with Islam, and therefore was dubious.

Apparently there was a huge decline in soap use in Europe in the medieval period.

Which, of course, would allow the louse population to flourish.

Racism killed a third of Europe in the 13th century. Silly buggers.

By the way, I know we are increasingly encouraged to cold-wash our clothes these days for environmental reasons, but maybe ... let's not, eh?