(CNN) Doctors have shared new details about the first case of Wuhan coronavirus in the United States in a paper published Friday in the New England Journal of Medicine.

In the new report, doctors describe how the man progressed from initially mild, nonspecific symptoms to pneumonia on the ninth day of his illness.

The patient -- a 35-year-old resident of Snohomish County, Washington, with no history of major health problems -- had returned from visiting family in Wuhan on January 15. He had not visited the seafood market where a number of early patients were initially linked, nor did he have any known contacts with sick people during his visit.

Still, the man had seen a health alert by the US Centers for Disease Control and Prevention and decided to visit an urgent care clinic on January 19, at which point he had been coughing for four days.

Doctors initially found nothing abnormal on chest X-rays, but sent swabs to the CDC because of his recent travel to Wuhan. By the following day, on January 20, they confirmed the man had the novel coronavirus.

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