"his men spread fear abroad like the Black Death"
The Black Death is the name given to the first and worst outbreak of the bubonic plague, an illness which regularly swept through Europe from the 14th until the 17th centuries. The black death arrived in Europe from Asia in 1348, and in just two years wiped out around 30% of the continent's entire population. The death toll was even higher in countries like Britain, where it killed up to 50% of people and left entire villages dead.
Plague is extremely contagious and is usually spread through flea bites- it spread through Europe with the help of rats, who would carry infected fleas from place to place and then die, meaning that the fleas would transfer themselves to nearby humans and infect them instead. Bubonic plague kills 75% of its victims within 8 days, with the other, rarer forms of plague working even faster.