Scientists are not yet able to say how deadly the new coronavirus really is. As the virus spreads around the world, the percentage of patients dying differs from one place to the next, even within China.

Other countries are now facing a growing number of cases of the virus that causes the disease, called COVID-19. Yet a low death rate can still mean a lot of victims. So, scientists are trying to understand why death rates are high in some areas and low in others.

"You could have bad outcomes with this" until health officials really understand how to control the disease, Bruce Aylward warned this week. Aylward is a doctor and a World Health Organization (WHO) envoy who led a team of scientists just back from China.

The new coronavirus first appeared in the city of Wuhan, in central China.

There is no special treatment for COVID-19, but medical care received as early as possible may help. As the danger of the virus became clear, Chinese hospitals moved quickly to admit patients.

Aylward expressed anger at people who say the death rate is not that bad because of the large number of mild cases.

Until the past week, most people diagnosed with the virus outside of China had become infected during a visit there.

People who travel a lot are often healthier and may be better able to recover, notes Lauren Sauer of the Johns Hopkins Bloomberg School of Public Health. And as countries began examining returning travelers for the virus, they found infections far earlier.

That is now changing, with coronavirus cases increasing in Japan, Italy and Iran.

Aylward warned that officials should be careful of "artificially high" death rates early on. Some of those countries likely are identifying the sickest patients first and missing milder cases.

How does COVID-19 compare to other diseases?

A different form of this new virus caused severe acute respiratory syndrome (SARS), a far deadlier infection, in 2003. About 10 percent of SARS patients died.

Influenza is a different virus family. Some forms of the flu are deadlier than others. On average, the death rate from flu is about 0.1 percent, notes Anthony Fauci, head of the United States' National Institute of Allergy and Infectious Diseases.

That death rate is far lower than what has been calculated so far for COVID-19. Millions of people get the flu every year around the world, leading to hundreds of thousands of deaths every year.

Who is most at risk from COVID-19?

Older adults who suffer from conditions like heart or lung disease are more at risk than other individuals. Deaths are much less common among younger patients, Aylward said.

The sickest to start with are at the highest risk of death, Aylward said. He added that a small number of those with mild infections do go on to die. No one understands the reason.

The World Health Organization says people with mild cases recover in about two weeks. The recovery time for those who are much sicker can take up to six weeks.

I'm Susan Shand.

The Associated Press reported this story. Susan Shand adapted it for VOA Learning English. George Grow was the editor.