A case in Oregon is a rarity in the United States. It is not expected to spread. But this ancient and deadly disease remains a threat around the world.
For centuries, scientists and historians have wondered where the Black Death — the deadliest pandemic in recorded history — came from. New research sheds light on the ancient disease.
That's when a vaccine for plague was invented — and authorities began to consider requiring proof of vaccination before visiting pilgrimage sites in India. The debate has raged ever since.