The new virus is from the coronavirus family, which includes those viruses that can cause the common cold, as well as more serious illnesses such as the Severe Acute Respiratory Syndrome (SARS) and the Middle East Respiratory Syndrome (MERS).
Here is how the dangerous new virus compares with other deadly global epidemics.
On Wednesday, for a second day, the number of infections grew dramatically.
More than half of the cases in which symptoms began before Jan. 1 were tied to a seafood market, but only 8 percent of cases after that have been, researchers found. They reported that the average incubation period was five days.
In a report published Wednesday, Chinese researchers suggested that person-to-person spreading of the virus among close contacts occurred as early as mid-December. Based on the first 425 confirmed cases, the researchers estimated that each infection led to 2.2 others, on average. That’s a bit more than the ordinary flu but far less than some other respiratory diseases such as whooping cough and tuberculosis.
The World Health Organization’s emergencies chief said the few cases of human-to-human spread of the virus outside China -- in Japan, Germany, Canada and Vietnam -- were of “great concern” and were part of the reason the United Nations health agency’s director-general was reconvening a committee of experts on Thursday to assess whether the potential outbreak should be declared a global emergency.
Dr. Michael Ryan spoke at a news conference in Geneva after returning from a trip to Beijing to meet with Chinese President Xi Jinping and other senior government leaders there. He said China was taking “extraordinary measures in the face of an extraordinary challenge” posed by the virus.
To date, about 99 percent of the nearly 6,000 cases are in China.
Ryan estimated that the death rate of the new virus is at 2 percent, but said the figure was very preliminary.
Ryan noted that there were several aspects of the new virus that are extremely worrying, citing the recent rapid spike in cases in China. He said that while scientists believe the outbreak was sparked by an animal virus, it’s unclear if there are other factors driving the epidemic.
“Without understanding that, it’s very hard to put into context the current transmission dynamics,” he said.
SARS / MERS
The new virus comes from a large family of coronaviruses, some causing nothing worse than a cold. But in late 2002, a coronavirus named SARS erupted in southern China, causing severe pneumonia that rapidly spread to other countries. It infected more than 8,000 people and killed 774 -- and then it disappeared, thanks to public health measures.
In 2012, another coronavirus dubbed MERS began sickening people in Saudi Arabia. It’s still hanging around, causing small numbers of infections each year. The World Health Organization has counted nearly 2,500 cases of MERS in the Middle East and beyond, and more than 850 deaths.
SARS and MERS came from animals, and this newest virus almost certainly did, too. The first people infected with the coronavirus visited or worked at a seafood market in the Chinese city of Wuhan.
SARS was initially traced to civet cats sold in a live animal market, but scientists later decided it probably originated in bats that infected the cats. People can catch MERS from infected camels, although again, bats likely first spread that coronavirus to camels, too.
The animal-to-human jump is a huge concern for all kinds of viruses. Every so often, new strains of bird flu make the jump from Asian live poultry markets to people, for example.
The new virus has now infected more people in China than were sickened there during the 2002-2003 SARS outbreak.
On Wednesday, the number of cases jumped to 5,974, surpassing the 5,327 people diagnosed with SARS, a cousin of this new virus.
The death toll, which rose to 170 on Wednesday, is still less than half the number who died in China from SARS.
The SARS virus killed about 10 percent of people who caught it.
The Associated Press contributed to this report.