The number of new cases has risen sharply in China, the center of the outbreak. There were 440 confirmed cases as of midnight Tuesday in 13 jurisdictions, said Li Bin, deputy director of the National Health Commission. Nine people have died, all in Hubei province, since the outbreak emerged in its provincial capital of Wuhan late last month.
"There has already been human-to-human transmission and infection of medical workers," Li said at a news conference with health experts. "Evidence has shown that the disease has been transmitted through the respiratory tract and there is the possibility of viral mutation."
The illness comes from a newly identified type of coronavirus, a family of viruses that can cause the common cold as well as more serious illnesses such as the SARS outbreak that spread from China to more than a dozen countries in 2002-2003 and killed about 800 people.
Thailand authorities Wednesday confirmed four cases, a Thai national and three Chinese visitors. Japan, South Korea, the United States and Taiwan have all reported one case each. All of the illnesses were of people from Wuhan or who recently had traveled there.
"The situation is under control here," Thailand Public Health Minister Anutin Charnvirakul told reporters, saying there are no reports of the infection spreading to others. "We checked all of them: taxi drivers, people who wheeled the wheelchairs for the patients, doctors and nurses who worked around them."
Travel agencies that organize trips to North Korea say the country has banned foreign tourists because of the outbreak. Most tourists to North Korea are either Chinese or travel to the country through neighboring China. North Korea also closed its borders in 2003 during the SARS scare.
Other countries have stepped up screening measures for travelers from China, especially those arriving from Wuhan. Worries have been heightened by the coming of the Lunar New Year holiday rush, when millions of Chinese travel at home and abroad.
Officials said it was too early to compare the new virus with SARS or MERS, or Middle East respiratory syndrome, in terms of how lethal it might be. They attributed the spike in new cases to improvements in detection and monitoring.
"We are still in the process of learning more about this disease." Gao Fu, an academician of the Chinese Academy of Sciences and head of the Chinese Center for Disease Control said at the same news conference.
Gao said officials are working on the assumption that the outbreak resulted from human exposure to wild animals that were being traded in illegally at a food market in Wuhan and that the virus is mutating.
Health officials confirmed earlier this week that the disease can be spread between humans after finding two cases of people in southern Guangdong province who had not been to Wuhan. It's not clear how contagious it is, but person-to-person transmission could allow it to spread more widely.
The Lunar New Year is a time when many Chinese return to their hometowns to visit family. Li, the health commission official, said that measures were being taken to monitor and detect infected people from Wuhan, and that people should avoid going to the city and people from the city should stay put for now.