Chinese health officials in the central city of Wuhan confirmed 136 new cases of a new coronavirus - a huge spike - over the past three days.
The Wuhan Municipal Health Commission says the total number of cases of the virus now exceeds 200, including two new cases in Beijing and one in Shenzhen in southern China. Most of the confirmed cases are described as mild, but three deaths have been reported.
Doctors in Wuhan, China's seventh most populous city, have stepped up screening for suspected cases of pneumonia. They are urging people to be more conscious of their personal hygiene and to cover their mouths when they cough or sneeze.
On Friday the U.S. Centers for Disease Control and Prevention started screening passengers arriving from Wuhan at three airports - San Francisco, Los Angeles, and New York. Airports in Japan, Thailand, South Korea and Singapore are also screening passengers.
Passengers on a flight that arrived Saturday morning in San Francisco said they went through the screening and it was an easy procedure. Their temperature was taken and they filled out a form.
Chinese and U.S. health officials are particularly concerned because many of the 1.4 billion Chinese citizens are expected to travel for the Lunar New Year holiday that starts Jan. 25, both inside China and beyond.
A coronavirus is one of a large family of viruses that can cause illnesses ranging from the common cold to the deadly Severe Acute Respiratory Syndrome. SARS, which also started in China, killed nearly 800 people globally during an outbreak 17 years ago.
Chinese health experts know little about the new strain, dubbed 2019-nCoV, in Wuhan, especially how it is transmitted. They suspect the outbreak started in a Wuhan seafood market, which also sold other animals such as poultry, bats, marmots and wild game meat, but some patients say they were never there.
Health officials are urging caution but say there is no reason to panic. The World Health Organization is not recommending against travel to China, and China's National Health Commission says the current outbreak is "preventable and controllable."
According to the latest information received and WHO analysis, there is evidence of limited human-to-human transmission of the virus, the WHO tweeted Sunday. This is in line with experience with other respiratory illnesses and in particular with other coronavirus outbreaks.
"While there is currently no clear evidence of sustained human-to-human transmission, we do not have enough evidence to evaluate the full extent of human-to-human transmission. This is one of the issues that @WHO is monitoring closely."
Of the new cases announced this weekend, all involve adults ages 25 to 89. About half are male (78) and half are female (75), according Minnesota Center for Infectious Disease Research and Policy, which translated the Wuhan commission's statement.
Of the 198 patients confirmed so far, 28 have recovered or been discharged. Of the 170 people still in the hospital, 126 have mild illness, 35 are listed as severe, and 9 are in critical condition. Three deaths have been now reported. Hospitalized patients in Wuhan are isolated at a designated facility.
The number of close contacts under monitoring has risen from 763 to 817, and monitoring is still under way for 90. So far no related cases have been found in contacts.