From an historical perspective plagues have destroyed more armies than have opposing armies. This has been a common thread throughout history ever since the urban revolution of Neolithic times. Plagues feed off of people in densely crowded cities beginning around 6000 BCE. If you want to avoid a plague, in other words, stay away from crowds. Eventually a plague will run its course and die out, but the population will have been greatly decreased. In time, another plague, however, will follow as people will over and again flock into densely crowded spaces. Plague germs are ever evolving organic microbes that kill their host, but must have an endless body of hosts to move onto since they are killers by nature. In other words, they cannot control the fact that they are flawed in that they cannot stop killing a host and only survive as long as they can spread their off-spring to another host--or victim.