retrovirus

The company also inactivated retroviruses that pigs carry and could be harmful to humans.

"If we didn't have retroviruses sticking their sequences into the vertebrate genome, then myelination wouldn't have happened, and without myelination, the whole diversity of vertebrates as we know it would never have happened."

However, most retrovirus integrations are very old, already degraded and therefore inactive -- their initial impact on host health has been minimised by millions of years of evolution.

"If you get infected, you've already lost the game because it's a retrovirus," Bennett agrees.

"It turned out that the antibodies are recognising remnants of what's termed endogenous retroviruses," Prof Julian Downward, an associate research director at the Francis Crick Institute, told me.