The deadly MERS virus that killed 38 people three years ago has returned to Seoul.
A man was diagnosed with the disease on Saturday after returning from Kuwait.
Worried that South Korea could see a repeat of the 2015 outbreak, the government is under emergency alert, trying to tackle the virus quickly and accurately to prevent it from spreading.
The government has assigned officials to monitor the 439 people that were on the same flight as the patient and check for any symptoms.
"This time the people on the same flight will be under active monitoring. They will not be isolated in their homes, but their designated officials will check their health every day."
One English woman who was on the same plane as the MERS patient was tested for infection because she had a fever.
The first examination showed she was not infected, but the Ministry of Health and Welfare will put her through a second examination before discharging her from hospital.
Meanwhile, one South Korean citizen in Kuwait who was working for the man diagnosed with MERS in Seoul took a medical examination in Kuwait after showing symptoms of MERS.
However, the result of the examination showed the person was not infected.
Eleven other Koreans in Kuwait concerned of having caught MERS also had medical checkups but were found to have not contracted the virus.
The South Korean Minister of Health and Welfare said they are tracking down MERS patient's whereabouts for the past days in Kuwait, and that Seoul is working with the government of Kuwait, to take safety measures if anyone shows symptoms of MERS.
Hong Yoo, Arirang News.