South Korean officials reported on Friday (April 10) that at least 91 patients thought cleared of the new coronavirus had tested positive again.
Korea Centers for Disease Control and Prevention have said the virus may have been "reactivated"rather than the patients being re-infected.
But authorities say it remains unclear what is behind the trend - and that investigations are still underway.
Some experts have suggested that false test results could also be at fault.
Or that remnants of the virus could still be in patients’ systems - but may not be infectious or of danger to the host or others.
But the prospect of people being re-infected with the virus is of international concern.
Many countries are hoping - and relying - on infected populations developing sufficient immunity to prevent a resurgence of the virus.
South Korea on Friday reported its lowest number of new infections after daily cases peaked at more than 900 in late February - and the death toll rose to at least 211.