The Bubonic Plague (Black Death) was spread by fleas and rodents. This is because it can turn your fingers a blue/black color. The black death led to the decline of feudalism because so many people died, and led to many changes in Europes economic and social structures.
The growth of towns and cities started when a lot more people came from all over to trade and to get the goods they needed. The growth of towns and cities resulted in the decline of feudalism because people left their manor and left and went into the cities to buy goods at markets.