The Origins of the Black Death
The main causes of the devastating Black Death in Europe were actually fleas that were the carriers of the bubonic plague, which was spread by the black rats that they lived upon. Had the fleas and the black rats carrying them had not reached Europe then the Black Death would not have happened.
These fleas and black rats were not naturally found in Europe during the 13th century, yet they had already spread the Black Death through large areas of Asia. The fleas carrying the bubonic plague and inadvertently moved by invested black rats would have taken a much longer period to reach Europe itself via land from Asia. However the demographic collapse brought about by the Black Death was unintentionally introduced to the continent via sea to the European ports that traded with China and also India.