Dead lung cells could be causing people to test positive twice for Covid-19, it comes as governments around the world try to ease coronavirus restrictions without causing a second wave.
The World Health Organisation (WHO) says all countries must remain alert for a second wave.
Singapore has rushed to install 4000 new hospital beds after their infection rate spiked again after it was believed to be under control.
WHO says testing will be crucial in tracking the virus and tracing any new outbreaks.
Those needing hospital care for Covid-19 is falling, a key sign that coronavirus cases are decreasing.
However, scientists warn there isn't much opportunity to relax the lockdown measures without prompting a spike in cases.
The coronavirus rate of infection currently sits at 0.7, below the threshold of one meaning where an infected person would only infect one other.
Scientists say schools opening may add another 0.2 and lifting the lockdown entirely could add two to the rate of infection.
From tomorrow, people in Spain will be required to wear masks on public transport.
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