The correct answer is - Trade routes.
The lines with golden color are representing the trade routs on this image. The trade routes depicted are the ones that connected Asia and Europe, most of which are part of the Silk Road, which unlike the popular belief that it was a single route, it was actually a network of trade routes.
While the trading routes were very good in the sense of trade, economic development, sharing of ideas, cultures, communication, they also brought something devastating through them, the plague.
The plague is though to have started somewhere in the Chinese ports. Through the trade, a flea that carried the disease was also transported, so people started to get infected by it.
The disease was spreading very quickly, and over a very big area, killing millions of people.
