Quarantine authorities in South Korea have begun monitoring a mysterious inflammatory disease affecting children and teens that is believed to be linked to COVID-19.

The illness is called pediatric multisystem inflammatory syndrome(PMIS), and has been spreading in the United States and Europe. As of Saturday, it has hit 13 countries since the first reported case in Europe last month.

Jeong Eun-kyeong, head of the Korea Centers for Disease Control and Prevention(KCDC), said in a briefing that authorities will start a system on Monday to monitor and trace the illness, including drafting procedures on how cases are to be reported.

The illness is similar to Kawasaki disease which causes inflammation in children four years and younger. In the worst case, PMIS could lead to death, though the exact cause is unknown. The oldest reported patient was a 20-year-old from the U.S.

Symptoms include a fever of 38 degrees or higher that continues for more than 24 hours. Blood tests may confirm inflammatory symptoms. Patients, aged 19 or younger, must have been previously infected or exposed to COVID-19.