Some viruses are so extraordinarily lethal that they’ve never managed to spread throughout the human population—see ebola, which I believe has approximately a 90% lethality, and even the survivors are severely brain-damaged—because they harm their victims so rapidly that they don’t have a chance to spread.
Our modern transportation systems have effectively eliminated most of the barriers between human populations. All of our eggs are in one basket. If a highly lethal virus that will spread throughout an entire population and kill it arises, that basket will be dropped.
Simply put, it doesn’t.
Some viruses are so extraordinarily lethal that they’ve never managed to spread throughout the human population—see ebola, which I believe has approximately a 90% lethality, and even the survivors are severely brain-damaged—because they harm their victims so rapidly that they don’t have a chance to spread.
Our modern transportation systems have effectively eliminated most of the barriers between human populations. All of our eggs are in one basket. If a highly lethal virus that will spread throughout an entire population and kill it arises, that basket will be dropped.