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One source of rumours was a paper posted by scientists in India claiming that short insertions in the viral genome had an “uncanny similarity” to HIV. Although the paper was quickly withdrawn, its allegations live on in social media.

The research was “wrong on many levels,” said Dr. Bedford, whose lab studies the evolution of viruses. The genes it shares with HIV are extremely short sequences naturally shared by other organisms and “repeated again and again throughout the tree of life.”

He also disputed claims that Covid-19 might have infected humans from snakes or even fish. The most likely scenario, based on genetic analysis, was that the virus was transmitted by a bat to another mammal between 20-70 years ago.

This intermediary animal — not yet identified — passed it on to its first human host in the city of Wuhan in late November or early December 2019.

Dr. Bedford is a leader of the worldwide Nextstrain collaboration that began to analyze Covid-19 genomes when they were released in January by Fudan University and the Chinese Centre for Disease Control. By now scientists around the world have published the genetic sequence of virus taken from about 100 patients.

Photo by HECTOR RETAMAL/AFP via Getty Images

They show mutations taking place at a slow pace as the infection passes from person to person.

Typically the virus in one patient today is different in around five of the 30,000 biochemical letters of its genetic code, but these are random changes rather than any sign that it is becoming more virulent or infectious, Dr. Bedford said.