Here’s how that would work. The virus that causes COVID-19 has a genome based on RNA, which like DNA, is made up of a sequence of chemical “letters” called nucleotides. As the coronavirus invades people’s cells and multiplies, it naturally makes mistakes when replicating its genetic code, swapping a few of those letters. “Imagine having to copy 30,000 letters by hand,” says Stephanie Spielman, assistant professor of biological sciences at Rowan University. ”You might make a mistake, and that’s what a mutation is.”

Once a mutation occurs, it remains in future copies, creating a lineage called a variant. Just swab a bunch of infected noses, compare the variants, and you can trace the origins.

“If you have clusters of people with similar sequences, that’s a strong indication they had a common source,” says Joshua Michaud, associate director for Global Health Policy at the Kaiser Family Foundation. This procedure could even narrow down where in the country Trump’s contingent caught the coronavirus. (National Geographic contacted the White House for comment but received no reply.)