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

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

Experts say the effects appear to depend on how robust or weakened a person’s immune system is. Older people or those with underlying health issues, like diabetes or another chronic illness, are more likely to develop severe symptoms.

Xiao conducted pathological examinations of two people in China 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, Xiao said the records showed.

The other patient, a 73year-old man, was somewhat healthier, with a history of hypertension that he had managed well for 20 years. 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 the coronavirus.

Xiao said that the man had almost certainly been infected during his first stay in the hospital, since other patients in his post-surgical recovery room were later found to have the coronavirus. Like many other cases, it took the man days to show respiratory symptoms.

The man recovered after 20 days in the hospital’s infectious disease unit. Experts say that when patients like that recover, it is often because the supportive care — fluids, breathing support and other treatment — allows them to outlast the worst effects of the inflammation caused by the virus.