About 116 former coronavirus patients have tested positive for the second time.
They tested negative when discharged from hospitals, according to the Korea Centres for Disease Control and Prevention (KCDC).
They then tested positive days later.
The strange development is now under investigation by the KCDC and the World Health Organisation.
Several theories have been floated to explain the phenomenon, which has also been reported in China.
One, that the re-occurrence of the virus could indicate that the discharged patients failed to build up immunity after they had it.
Two, that the patients in South Korea that tested positive may not have been reinfected.
It may be that the virus is instead “reactivating,” with the patients experiencing something more like a relapse than a second case.
The WHO has told multiple news outlets that it is aware of the 116 patients in South Korea and is investigating.
The trend has also been reported in China: people who have gotten the virus, recovered, tested negative and then tested positive again.
The WHO suggested that this could just all be the result of faulty testing.
“It is important to make sure that when samples are collected for testing on suspected patients, procedures are followed properly,” the WHO said.