Before the Black Plague, most of the people in Medieval Europe were peasants. Peasants usually lived in houses that were made of wood or mud, with basically no furniture. Their houses were also homes for the black rats, which lived in their piles of garbage or on the thatched roof. People usually didn't try to get rid of the rats, because the rats helped finish off the scraps of food on the floor, and they didn't know that the rats had fleas that could give them the Black Plague. The towns backed then were a maze of dirty, narrow, streets that was filled with beggars, merchants, friars, pilgrims, soldiers and jugglers. Most of the garbage that people had was all poured into the drains, which lead to the local river. Rats and animals would come to the river and eat the garbage.
Europe's population had increased a lot during the 13th century, but Europe didn't make enough food, so there was a lot of famines. Physicians and apothecaries had cures for people, but their methods were not effective and usually made the person more worst. The Mongols saw that by taking over Genoa's ports that they would be wealthy. In 1347 the Mongols attacked the town with catapults, but the Genoese didn't surrender. The attack went on for months until the Mongol commander thought up a plan. The plague had already gotten some of his soldiers, and they were afraid that the dead bodies would infect the other soldiers so the commander's plan was to fire the dead bodies over the castle walls, which made the people, catch the plague and then they took over. But the plague spread, because the ships continued to trade carrying goods to Italy. In Italy they found out that the plague was coming from the ships so they didn't allow the ships to trade, so instead they went to France, then the plague spread to Europe. The plague all started in Asia and then it spread all over the world.
At the beginning it killed mostly people that lived by the ports, but then it moved inland and killed more people.