Opinion | Where Did the Coronavirus Come From? What We Already Know Is Troubling.

All this leaves a lot of possibilities open and a lot of confusion.

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Since most pandemics have been due to zoonotic events, emerging from animals, is there reason to doubt lab involvement? Maybe if you look at all of human history. A better period of comparison is the time since the advent of molecular biology, when it became more likely for scientists to cause outbreaks. The 1977 pandemic was tied to research activities, while the other two pandemics that have occurred since then, AIDS and the H1N1 swine flu of 2009, were not.

Plus, once a rare event, like a pandemic, has happened, one has to consider all the potential paths to it. It’s like investigating a plane crash. Flying is usually very safe, but when a crash does happen, we don’t just say mechanical errors and pilot mistakes don’t usually lead to catastrophes and that terrorism is rare. Rather, we investigate all possible paths, including unusual ones, so we can figure out how to prevent similar events.

Perhaps the biggest question has been what to read into the location of the outbreak, a thousand miles from the closest known viral relatives yet close to a leading research institution.

Sometimes the curiosity around the location has been waved away with the explanation that labs are set up where viruses are. However, the Wuhan Institute of Virology has been where it is since 1956, doing research on agricultural and environmental microbiology under a different name. It was upgraded and began to focus on coronavirus research only after SARS. Wuhan is a metropolis with a larger population than New York City’s, not some rural outpost near bat caves. Dr. Shi said the December 2019 outbreak surprised her because she “never expected this kind of thing to happen in Wuhan, in central China.” When her lab needed a population with a lower likelihood of bat coronavirus exposure, they used Wuhan residents, noting that “inhabitants have a much lower likelihood of contact with bats due to its urban setting.”

Still, location itself is not proof, either. Plausible scenarios implicating research activities don’t rule out other options.

This week, Jesse Bloom, an associate professor at the Fred Hutchinson Cancer Research Center, told me that when he recovered and analyzed a set of partial early Wuhan genetic sequences that had been removed from a genomic archive, it supported “substantial existing evidence that SARS-CoV-2 was circulating in Wuhan prior to the seafood market outbreak.” Both the early reports from Chinese scientists and the more recent W.H.O. investigation this winter found many of the early cases had no connection to the seafood market, including the earliest acknowledged case so far, on Dec. 8, 2019. So the seafood market may not have been the original location of the outbreak.

It’s also plausible that an outbreak could have started someplace else and was detected in Wuhan simply because it was a big city. Testing blood banks from across China, especially in areas near wildlife farms and bat caves, would help, but with limited exceptions, the Chinese government has not carried out such research — or allowed the sharing of the results if it has.