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. 

The phrases that convey the ironic tone are:

My former speeches have but hit your thoughts,Which can interpret farther. The gracious Duncan Was pitied of Macbeth: marry, he was dead.

Explanation:

This question is about "Macbeth" a play written by Shakespeare that shows how Macbeth usurped the throne of King Duncan who was also his personal friend and the consequences that this attitude led.

Lennox's speech is a great example of verbal irony, where Lennox is extremely sarcastic while presenting the hostility he felt towards the nobles who were listening to him.

We can see this sarcasm in the lines:

My former speeches have but hit your thoughts, Which can interpret farther: Where Lennox says that the nobles can interpret his words, however they want, however, he knows that there is only one conclusion and one interpretation of his speech, which means that the nobles agree with all his words. The gracious Duncan Was pitied of Macbeth: marry, he was dead: Where he mocks Macbeth's suffering and grief over Duncan's death. He claims that Macbeth suffered so much for the death of his friend, that he killed him himself, so that he could have a worthy mourning.