Which of the following most contributed to the plague of 1348

In Medieval England, the Black Death was to kill 1.5 million people out of an estimated total of 4 million people between 1348 and 1350. No medical knowledge existed in Medieval England to cope with the disease. After 1350, it was to strike England another six times by the end of the century. Understandably, peasants were terrified at the news that the Black Death might be approaching their village or town.
The Black Death is the name given to a deadly plague (often called bubonic plague, but is more likely to be pneumonic plague) which was rampant during the Fourteenth Century. It was believed to have arrived from Asia in late 1348 and caused more than one epidemic in that century – though its impact on English society from 1348 to 1350 was terrible. No amount of medical knowledge could help England when the plague struck. It was also to have a major impact on England’s social structure which lead to the Peasants Revolt of 1381.
Up until recently the Black Death was thought to have been caused by fleas carried by rats that were very common in towns and cities. When the fleas bit into their victims, it was thought they were literally injecting them with the disease.