About every 100 years there is some type of pandemic that kills a certain percentage of the world population. There are various causes of pandemics from killing cats in the middle ages to eating species of animals not fit to eat.
The Jet Stream is a river of high speed air that circulates around the globe. It keeps the frigid arctic air in Canada and the northern tier states. A normal winter this is exactly what happens. Arizona is usually warm and dry when the jet stream stay in the north. Years when the Jet Stream drops into the southwest we enjoy a cold winter.