South Korea reported on Friday that 91 recovered coronavirus patients have tested positive for the disease again, raising questions over health experts' understanding of the pandemic.

The prospect of people being re-infected with the virus is of international concern, as many countries are hoping that infected populations will develop sufficient immunity to prevent a resurgence of the pandemic.

The reports have also prompted fears the virus may remain active in patients for much longer than was previously thought.

Korean health officials reported Friday that 91 patients thought to have been cleared of the virus had tested positive again, up from 51 people on Monday.

The Korea Centers for Disease Control and Prevention (KCDC) announced it had sent a team to the city of Daegu, the worst hit area, to investigate why patients there were testing positive again.