A man in the United States has been infected with Covid twice, with the second infection becoming far more dangerous than the first, doctors report.
The 25-year-old needed hospital treatment after his lungs could not get enough oxygen into his body..
Scientists say the patient caught coronavirus twice, rather than the original infection becoming dormant and then bouncing back.
A comparison of the genetic codes of the virus taken during each bout of symptoms showed they were too distinct to be caused by the same infection.
He said even people who have recovered should continue to follow guidelines around social distancing, face masks and hand washing.
As countries endure a second wave of the virus, we may start to get clearer answers.
It had been assumed that a second round of Covid would be milder, as the body would have learned to fight the virus the first time around.
It also remains possible that the initial immune response made the second infection worse. This has been documented with diseases like dengue fever, where antibodies made in response to one strain of dengue virus cause problems if infected by another strain.