• One theory about why the Black Death happened in the fourteenth century was that an area in China, in 1346, saw a large Black Rat population being forced to leave its home and bring them into contact with humans. When the rats had a sudden die off, the fleas jumped to human hosts. Then, so the theory goes, the humans brought the plague with them along the East-West trade routes.
  • David Hurley: was a great medieval historian who raised questions about the Black Death. He noticed that nowhere or no one in medieval Italy mentioned a mass die-off of animals which carried the plague (which were noticed in other incarnations of terrible plagues). Hurley also noticed that unless the plague was pneumonic, trade routes couldn’t explain how the plague spread so quickly. Plague also seemed to move in seasonal cycles; epidemiologically, it seems that Winter should have been the worst time for the plague to spread instead of the period when it leveled off since people and rats would be crowded together. Instead, there are factors which suggest that other diseases impacted Western Europe alongside the bubonic plague. For example, in Florence Italy in 1377– after the first large bout of plague subsided– those who prepared bodies for burial wrote in the Book of Death about whether victims had “the sign,” and so they thought that these more recent victims didn’t have a new disease but rather the old plague. But, what was the sign? It varied.
  • One theory postulates that naturally occurring anthrax might have been responsible for the deadly nature of the death. Passed from animals once they have absorbed the spores to humans once the meat of the dead animal has been consumed, the spores can be highly lethal (with symptoms that sound a lot like the plague). This would make sense since in a time before established veterinary systems, animal infections would be overlooked.
  • Another theory posited in 1979 by Fred H. was that the plague came from space. The theory throughout his and his collaborators books goes like this: Darwin’s theory is correct up to a point but the conditions on Earth are not enough o develop fauna and the sort. For life to advance as it has what was needed was something external. The name for this is Pan-Spermia, or that the seeds of life exist throughout the universe and that they move throughout space via comets and asteroids. This is called Vertical Transmission. This also includes disease. And, in the case of diseases, they can be spread along these same vertical transmissive routes until traditional horizontal transmission– organism to organism– takes place. Supposedly, this theory explains why the plague appeared between the centuries with such long gaps between outbreaks, because it came from space. (Mainstream science doesn’t have a lot to say about this; it is a fringe theory that can’t be proved.)
  • One more reliable theory published in a highly respected academic journal is that the plague outbreaks correspond to prime gerbil breeding seasons in the far east. These gerbils moved West and were able to survive the various climes that rats and fleas could not. There are holes in this theory but no single theory can yet accurately explain why the plague appeared, moved, and spread as it did.
  • Another theory that explains why more recent versions of the plague were not as virulent as the medieval variety is that the plague evolved into a weaker version since the two earlier versions burned themselves out by killing its hosts. In other words, the strain of plague that ravaged the middle ages is extinct. (The study was done by examining skeletons of plague victims throughout Western Europe and analyzing the proteins from dental enamel.)


  1. The space theory is whack pseudo-science by any means; in what ways is this theory similar to other theories about germs and virus that we see today in the age of anti-Vaxxers?
  2. With the vast number of theories about how the plague spread and established itself, do you feel there is one theory above others that is more likely? Do you feel that major possiblities are being left out?

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