1)<span /> The main cause of population decline in this period was the bubonic plague. This disease occurred unexpectedly and nobody knew how to cure it. Even after many years, vaccines have not been found to prevent plague. It was usually spread by rats, fleas, and Mongols. This disease killed a third of europe's population in a short period of time. As a result, people become very desperate regarding church as it was not helpful and stopped going there.
