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Scientists are still trying to figure out a how highly-transmissible variant of the coronavirus currently spreading through a growing number of countries came to be. But they suspect a single COVID-19 patient may be to blame.

Typically, coronavirus mutations are incremental in scale and fail to propagate or really take off, Wired reports. That made the B.1.1.7 variant — the highly infectious version of SARS-CoV-2 first identified in the U.K. — all the more confusing, as it contained 17 different mutations from the original strain that swept the planet.

"It was unusual in September to all of a sudden see [a variant] pop up that had 17 [changes]," Adam Lauring, an RNA virus researcher at the University of Michigan Medical School, told the magazine. "It suggested that something unusual happened."

Figuring out how all those mutations emerged at once remains a challenge, but now scientists believe that the entire process happened within a single patient who was sick for so long that the coronavirus had all the time it needed to transform itself into a significantly more transmissible and perhaps even deadlier version of itself.

Doctors have seen immunocompromised patients face extremely long-lasting coronavirus infections. They haven't identified patient zero of the B.1.1.7 variant, to be clear, but clinical microbiologist Ravi Gupta from the University of Cambridge told Wired that he saw similar mutations occur in a lymphoma patient who also had a 102-day-long battle against COVID-19 due to his weakened state.

"The whole time, their immune system is effectively beating [the virus] up. So the virus has a chance to learn how to live with the human immune system," Emma Hodcroft, a postdoctoral research at Switzerland's University of Bern who tracks pathogenic mutations, told Wired.