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To find out where a virus comes from, researchers compare it to other viruses to try to trace its origin. This leads to claims like SARS-CoV-2 is 91 or even 96% similar to other known viruses. But what does that really mean?

Tim Harford talks to the virus ecologist Marilyn J Roossinck.

(Photo: A Microbiology lab carries out sequencing of coronavirus samples. Credit: Javier Pulpo/Getty Images)