Origins of the plague
The plague is caused by Yersinia pestis, an enzootic common in populations of fleas. Carried by rodents the fleas spread as fast as the rodents. Rodents often sat on trade ships and trade carts which spread the plague, Which is why the plague was so widely spread. The plague is also airborne which meant it was easily transferred from human to human.
How would people react?
People would react terribly. There would be massive panic and a lot of the country would fall into quarantines and people would be fighting with people in the streets to protect themselves. AS soon as something this big would hit people would be moving out of the country, Barricading themselves inside their homes, Scavenging for supplies, And as well as mass media making it a huge deal and creating more mass panic. People would demand the government put people who are sick into special camps. As well as probably everyone wearing masks and protective clothing. Also people would tweet about it a lot.