Eighteen years ago, scientists began to suspect that the bacterium Helicobacter pylori is a frequent cause of peptic-ulcer disease. The microbe resides in the stomachs of roughly half the world’s people and has been considered an ancient stowaway, already present in our prehistoric ancestors.
A study of H. pylori from 11 countries, however, suggests that the bacterium could be a relatively recent hitchhiker in people.
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