As a result, some patients may endure damage that is inflicted not just by the virus, but by their own immune system as it rages to combat the infection.

Experts have not yet documented whether the virus can affect the brain. But scientists who studied SARS have reported some evidence that the SARS virus could infiltrate the brain in some patients. Given the similarity between SARS and Covid-19, the infection caused by the new coronavirus, a paper published last month in the Journal of Medical Virology argued that the possibility that the new coronavirus might be able to infect some nerve cells should not be ruled out.

Why do some people get very ill but most don’t?

About 80 percent of people infected with the new coronavirus have relatively mild symptoms. But about 20 percent of people become more seriously ill and in about 2 percent of patients in China, which has had the most cases, the disease has been fatal.

The disease can seriously sicken adults of all ages. According to a report of the first recorded cases in the United States, young, previously healthy adults can develop severe symptoms that could require ventilators and other life support. These patients may have a better chance at survival. Older, frailer people, or those with underlying health issues, like diabetes or another chronic illness, face the greater likelihood of dying from the virus.

In China, Dr. Xiao conducted pathological examinations of two people who went into a hospital in Wuhan in January for a different reason — they needed surgery for early stage lung cancer — but whose records later showed that they had also had coronavirus infection, which the hospital did not recognize at the time. Neither patient’s lung cancer was advanced enough to kill them, he said.

One of those patients, an 84-year-old woman with diabetes, died from pneumonia caused by coronavirus, Dr. Xiao said the records showed.

The other patient, a 73-year-old man, was somewhat healthier, with a history of hypertension that he had managed well for 20 years. Dr. Xiao said the man had successful surgery to remove a lung tumor, was discharged, and nine days later returned to the hospital because he had a fever and cough that was determined to be coronavirus.