One question down, 15 to go. The Bubonic plague originated among high-immunity wild animals in the Himalayas, the central Asian steppes and Central and East Africa. After circulating without spreading to humans for a very long time, a severe climate change in the sixth century became the source of infection for the rest of the world.
Africa, rather than Asia, was most likely where the disease was first spread among humans. Evidence of this fact is apparent. The Asian population was infected by the Mediterranean through the Middle East. Persia, the Middle Eastern power had contracted the infection via Roman soldiers. In addition to this evidence a contemporary source, Evagrius, recorded that the plague originated in Africa.
The Bubonic Plague was most likely spread from humans to animals due to a severe change in the climate. Under certain climactic conditions (in this case a drought followed by heavy, excessive rainfall) the guts of fleas infected by the Bubonic plague were blocked. This was caused by clotted blood and the mixture of the disease’s bacteria. The fleas were infected by plague carrying-rodents, who are immune to the disease themselves. Due to the fact that their guts had become blocked, the fleas hunger became insatiable. The insects moved from one host to another, infecting each one and spreading the plague.
The rodents the fleas most likely inhabited were multimammate mice and gerbils. The multimammate mouse breeds very quickly when there is increased food available (such as there was following the climate change), therefore raising the amount of hosts of the plague. Due to the food conditions the population of the territorial gerbil was increased as well, feeding their need to find their own territory and forcing them to travel, carrying the disease with them.
It is probable that the two above mentioned rodents passed the disease onto the genus Arvicanthus, another rat-like being. In the wet conditions these creatures could have produced thousands of offspring each year, multiplying the number of hosts yet again. Following this, these rodents must have come into contact with the black rat, an animal known for invading human environments. The black rat is aggressive, adapts easily, eats almost anything and also is a very fast breeder under the right climactic conditions.
With the infected fleas now inhabiting the black rat, they traveled alarmingly quickly from the rodent to humans. As rat infested ships traveled from one trading port to another the plague spread, first along the East African coast all the way to Western Europe, until much of the antique world was infected.