The Gene That Saved Your Ancestors and May Now Be Attacking Your Intestines

Between 1346 and 1353, the Black Death killed somewhere between a third and half of Europe's population. This much is grimly familiar. What's less obvious is that the plague did what all catastrophic die-offs do: it edited the survivors' descendants.

The gene in question is called ERAP2. It comes in two flavours. One version produces a fully functional protein that helps the immune system chop up invading pathogens and wave the fragments at the body's defenders, essentially shouting here, kill this. The other version makes a shorter, less useful protein. When researchers compared DNA extracted from Londoners buried before, during, and after the plague — including the mass graves at East Smithfield, dug in a hurry for plague victims — they found that people carrying two copies of the functional ERAP2 variant were roughly 40 to 50 percent more likely to survive Yersinia pestis. In evolutionary terms, that's an enormous advantage compressed into a single generation.

So the survivors passed it on. The good version became more common. And here is the catch that makes the whole thing interesting: that same aggressive, trigger-happy immune calibration is now associated with Crohn's disease and other autoimmune conditions. The gene that helped your medieval forebears fight off the buboes may be the reason a modern descendant's immune system attacks the lining of their own gut.

An immune system is always making a wager. Respond too weakly and a real infection kills you. Respond too strongly and you start firing at things that were never a threat — pollen, food proteins, your own tissue. During a plague, aggression pays. In a world of antibiotics and sanitation, that same setting misfires, and the target becomes the house it was built to protect.

This is not the first time a survival trade has come due. The sickle-cell trait, carried by one copy of a mutated haemoglobin gene, protects against malaria; inherit two copies and you get sickle-cell anaemia. Evolution rarely offers a clean bargain — it offers whatever kept you alive long enough to reproduce, and lets the bill arrive later.

Worth noting: the plague study drew mainly on European remains, so the story of ERAP2 is, for now, a largely European one. But the underlying principle travels. The genome you carry is a ledger of every disaster your ancestors outlasted, and some of those old debts are still being collected on the inside of your intestines.