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The origin of the six-foot distancing recommendation is something of a mystery. “It’s almost like it was pulled out of thin air,” said Linsey Marr, an expert on viral transmission at Virginia Tech University.

When the virus first emerged, many experts believed that it was transmitted primarily through large respiratory droplets, which are relatively heavy. Old scientific studies, some dating back more than a century, suggested that these droplets tend not to travel more than three to six feet. This observation, as well as an abundance of caution, may have spurred the C.D.C. to make its six-foot suggestion, Dr. Marr said.

But that recommendation was not universal. The American Academy of Pediatrics recommends three to six feet of social distancing in schools, but the World Health Organization recommends just one meter, or 3.3 feet.