About 10 percent of the patients did not initially have the usual symptoms, cough and fever, but instead had diarrhea and nausea first. Other uncommon symptoms included headache, dizziness and abdominal pain.

Another cause for concern was that some patients who at first appeared mildly or moderately ill then took a turn for the worse several days or even a week into their illness. The median time from their first symptoms to when they became short of breath was five days; to hospitalization, seven days; and to severe breathing trouble, eight days. Experts say that pattern means patients must be carefully monitored, and it is not safe to assume that someone who seems to be doing well early on is out of the woods.

The finding is a “heads up” to doctors to keep an eye on these patients, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in a recorded interview posted by JAMA.

Like previous reports on coronavirus patients, this one found that older people and those with underlying health problems like diabetes, heart disease or cancer tended to become more severely ill than younger, healthier patients.

Over all, about 26 percent of the 138 patients needed intensive care; their median age was 66, compared with a median of 51 years for those who did not require intensive care.

For this series of patients, the death rate was 4.3 percent, which is higher than the estimates coming from other parts of China. The reason is not known, and the figures may change as more information is gathered.