I mentioned horses were a source of transportation, but camels in particular were very powerful sources of transportation, especially across Central Asia and as you get into the Middle East and Africa, because you have these large swaths of desert. Camels could not only carry a lot, but they had a lot of stamina, and they did not have to stop for water as frequently. In a lot of world history, we focus on the large, non-nomadic empires, like the Roman Empire and Han China. But as trade became more important, especially as you go through Central Asia, you could imagine that many of the people who were very good at trading, who were good at moving things, were people who were good at moving themselves, in particular, the nomadic people in Central Asia. Many of their cities along these trade routes became more and more wealthy and more and more powerful. Now, as I mentioned, goods were not the only thing that were transported along these routes. One of the not-so-good things was the spread of disease.