Scientists are working to learn more about whether someone who's been infected with Covid-19 has any immunity to the virus.
It's not only key in developing treatments and vaccines, but also in helping governments decide when to relax lockdown measures.
The big question currently, is are you immune to the virus if you've recovered from it?
A small study revealed that monkeys who'd had the virus didn't catch it again a month later, however it isn't as clear of a picture for humans.
"Early data's showing that there are antibodies being generated," viral immunologist Professor Mala Maini tells the BBC.
"But it's really important to follow up with more in-depth studies about whether these are the right kind, the neutralising antibodies.
"Those are the ones that can give protection."
Scientists wanting to find out how long that immunity last.
"From everything we know so far, it would be very hard for me to reassure you with any kind of certainty that if you've been exposed to the virus, if you'll definitely have protective antibodies in one, or two or three years' time," Professor Danny Altmann says.
Coronavirus can affect people in different ways, but does the severity of symptoms alter immunity?
Some people are so ill they end up in intensive care, while others have little to no symptoms. Scientists trying to understand what difference this makes.
"People who've got severely ill, make a lot of antibody in general, and people who haven't got so severely ill make a lot less," virologist Professor Wendy Barclay says.
"What that means may be that people who've only had a mild infection might lose their immunity faster than others, and might not be protected if the virus comes back."
Scientists say we need answers on immunity urgently, to determine the next steps to take in the fight against Covid-19.
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