At least four types of coronaviruses cause very mild infections every year, like the common cold. Most people get infected with one or more of these viruses at some point in their lives.

Another coronavirus that circulated in China in 2003 caused a more dangerous condition known as Severe Acute Respiratory Syndrome, or SARS. The virus was contained after it had sickened 8,098 people and killed 774.

Middle East Respiratory Syndrome, or MERS, first reported in Saudi Arabia in 2012, is also caused by a coronavirus.

The new virus has been named SARS-CoV-2. The disease it causes is called Covid-19.

How dangerous is it?

It is hard to accurately assess the lethality of a new virus. It appears to be less often fatal than the coronaviruses that caused SARS or MERS, but significantly more so than the seasonal flu. The fatality rate was over 2 percent, in one study. But government scientists have estimated that the real figure could be below 1 percent, roughly the rate occurring in a severe flu season.

About 5 percent of the patients who were hospitalized in China had critical illnesses.

Children seem less likely to be infected with the new coronavirus, while middle-aged and older adults are disproportionately infected.