The correct answer to this open question is the following,
In those years, people were ignorant. They were not educated, had no access to school. They were heavy influenced by the Catholic church and its dogmatic tenants. That is why many people believe that Black Death was a púnishment from God. People believe that as they were sinners, God had sent them the Bubonic plague that killed millions of people in Asia and Europe.
Historians think that in reality, the Black Death or Bubonic plague originated in Central Asia or Eastern Asia, and arrived in Europe due to trade. The firsts case were identified in Crimea in 1347. From then on, black rats that lived in the ships where products were transported, easily spread the disease all over Europe killing millions of people.