Plague of Justinian: 541-542
The Bubonic Plague is a disease carried by rodents and typically transfers to humans through fleas. This disease is common in rodent filled urban areas known as ‘plague focus’ or ‘plague reservoir.’ The Plague typically only spreads to humans if rodents are living in human habitation.
It takes about 14 days for the Plague to kill the contaminated rats making it less likely for fleas to gather; therefore, the rats need a new host. After three days of starving, the dying rats turn to humans. It takes 3-5 days for people to experience symptoms and get sick, and after another 3-5 days, 80% of people with the disease die. Therefore, from the introduction of the Plague to the rat colony, it takes about 23 days on average for the first person to die.