There are a few articles flying around today (including this BBC one: http://www.bbc.co.uk/news/science-environment-42690577) saying that the Black Death was spread by humans and not rats. To be honest, this sounds a bit clickbaity, and the actual results of the study say that the plague was spread by fleas, and not through interhuman transmission.
The epidemiology of the plague is complicated. Most people now agree that the Black Death was caused by a bacterium called Yersinia Pestis. The bacteria live in fleas, which are vectors, on rats, which are the hosts. If all the rats die of the disease in an epizootic, then the fleas need a new host. There are then three ways the disease can present: bubonic (with buboes), pneumonic (which looks like pneumonia) and septicaemic (which is the most deadly).
The new study says that they modeled three types of transmission: rats, airborne transmission and fleas and lice. Looking at the headline, I thought that the scientists would be arguing for airborne, interhuman transmission. This is problematic looking at modern interhuman transmission of Yersinia Pestis, which can be spread in the pneumonic form, although it’s a very ineffective way of spreading the disease.
What the scientists are actually saying is that the fleas and human body lice were responsible for spreading the plague. If we subscribe to the Yersinia Pestis model, then this actually makes a lot of sense. I’m not sure whether there would need to be a smaller mammalian host like rats first though, or some other rodent (one study talks about alpine marmosets spreading the disease). My main question remains as to how these fleas got the plague in the first place. Indeed, at the end of the BBC article, ‘reservoirs’ of disease in rodents are mentioned. This would suggest that the fleas lived initially on a rodent host, and then made their way to humans. This would mean that the presence of rats wasn’t necessary once the disease had spread to humans on fleas, to perpetuate its spread.
There is now lots of evidence that the Black Death of 1347-51 and indeed the plague in England in 1665 before the Great Fire of London were caused by Yersinia Pestis, and this rodent-flea-human transmission. However, I’m yet to be completely convinced that every single plague in the Premodern period was caused by this same bacterium. One of the big problems with this theory was that the rats which were most capable of spreading the plague were not prevalent in Europe during the time of the Black Death. However, other studies have proposed that other rodents were hosts. And, if this new study is correct, then once the fleas had moved from the rodent host to the human host, the presence of rats wouldn’t be necessary to facilitate the spread of plague.
Most importantly perhaps is the conclusion from the study- focusing on the importance of hygiene. The Black Death struck indiscriminately, but by 1665 and the Early Modern period, plague became a disease of the poor. This suggests that hygiene had a crucial role to play, whether it was stopping the ‘reservoir’ rats or rodents, or the personal hygiene which would prevent human lice and fleas. It’s important to remember that plague happened about every 10 years. It wasn’t just these big years of plague that everyone talks about. It was the fact that there was plague so frequently that is now being examined, and this is what had the major impact on society.
We still face epidemic disease today, and this is why people are so interested in finding out what happened in the past. For me, that’s why studying the human reaction to the disease is so interesting and important. We still don’t have all the scientific answers to explain disease, so people still often turn to religion when confronted with unexplainable epidemic outbreaks.
On the Trail of the Bianchi of 1399 