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.