One of our oldest companions is a microbe called Helicobacter pylori. It has been colonising our stomachs, and co-evolving with us, for the past 100,000 years. It was with us when were still confined to Africa, and came along for the ride as we spread throughout the world. You can actually trace how humans spread through places like the Pacific islands by comparing the strains of H.pylori in the stomachs of their modern descendants.

But in some parts of the world, this relationship breaks down. When the European conquistadors came to Central and South America, they brought their own strains of H.pylori with them, and many of these replaced the strains carried by native Amerindians.

This historical mismatch still persists today,