Language, religion, customs, holidays, beliefs, all of these things come to mind when we think about cultures colliding with each other. It's very easy to spread disease when people are interacting with each other from different parts of the world. You never know where people are from or what kind of people they live around. Disease is probably one of the scariest things when cultures meet and interact because there are a lot of diseases that you can see straight out and it's not visible, and the ones that aren't mostly visible are usually most contagious ones. This is one reason for the great loss of population in Europe and China.
The Bubonic Plague, which started in Greece and spread north to Europe, stayed there and spread more from 1347-1351. Most believe that the epidemic originated in rats in Asia. Infected rats were drawn to the food supplies of armies in China.
Fleas traveled from the rats to people, carrying the plague with them. The disease spread along the routes that traders and soldiers took to south Asia, southwest Asia, and even Africa. The Silk Road carried the plague and so did the wars. Mongolia and China both helped bring the plague when they were at war with Europe. After reaching the Black sea it was carried into the cities of Europe. Others believed that it was caused by an earthquake that released poisonous gasses, or God's anger. The more crowed and dirty the cities were the easier the plague was spread. And it did, it wiped out 1/3 of the population of Europe and 35,000,000 people in China. They were the most affected by the plague. Not only was the Bubonic plague widespread but there were other diseases being spread also.
One of these diseases was Small...