How did the black black death come?

The Black Death arrived in Europe around the 1320's. People traveled by sailing at the time, and those who traded or traveled caught the disease from infected rats that were aboard the ships. Originally the plague was known as the "Great Pestilence," it killed around 1/3 of Europe's population in two years. After the plague took its root in Europe, it moved about to reach new populations all over. The disease began due to a very pathogenic (rod shaped) bacterium. One bacterium is enough to kill a mouse. This bacterium is known as Yersinia pestis. This bacterium was carried by rats, however, the rats were not the main contributor. Fleas on the other hand, were the ones that transmitted the disease from rats to humans. All it takes is one flea to bite a mouse (they ingest the blood), and then bite into a human host, thus transferring the contaminated blood into the human. As the bacteria expands inside the flea, it congests their digestive tract, causing the flea to think it's starving. Naturally, the flea bites the new host more, but is unaware that it throws up the contaminated blood into the bite wound. This action spreads the plague quickly. As you can imagine, people of the time wanted to get rid of household rodents or the rats on their ships. Because they tried to kill them, the fleas were in close proximity of the humans, and the plague spread quickly.