The Korea Centers for Disease Control and Prevention said Sunday that a 54-year-old Korean man who'd been living in Wuhan has tested positive for the coronavirus.
The KCDC says the man showed no symptoms when he arrived at Incheon International Airport on Monday and was unaware that he had been infected.
But starting Wednesday, he had a fever, chills and muscle cramps.
His condition seemed to improve temporarily after he took some medicine, but when he got worse he contacted the health authorities on Saturday.
Since then, he's been under quarantine at Myongji Hospital in Gyeonggi-do Province.
The fact that the man was initially unaware of his infection raises concerns that he may have spread the virus before he was tested.
That brings the total number of confirmed cases in South Korea to three.
The first case was confirmed on Monday in a 35-year-old Chinese woman.
The second was confirmed on Friday in a South Korean man in his 50s.
The authorities say both of them are in a stable condition and are being treated at the Incheon Medical Center and the National Medical Center, respectively.
The KCDC says it's ramping up efforts to stop the virus from spreading.
"South Korea will designate all of China as a cronavirus 'contaminated zone' as of Tuesday. All travelers from China arriving in South Korea will have to turn in detailed health questionnaires.
Meanwhile, the South Korean government is reportedly considering using a chartered flight to bring back Korean nationals stranded in Wuhan.
About five hundred of them are reportedly in Wuhan, including some international students.
Some four hundred the the vast majority said they would like to take up the offer of a chartered flight.
South Korea's foreign ministry has not yet confirmed that plan, but is said to be discussing the matter with China.
The foreign ministry, also on Saturday, raised the travel advisory level for Wuhan to "red," the third highest in the four-tier warning system.
Authorities advise anyone who has a fever or respiratory problems within 2 weeks of traveling abroad to contact the disease control center by dialing 1-3-3-9.
People, they say, should also wash their hands regularly, wear a mask, and cover the mouth when coughing.
Kim Jae-hee, Arirang News.