Scientists have been investigating why some people get Covid multiple times while others seem able to keep dodging the virus despite exposure.
It emerged this week that Labour leader Sir Keir Starmer has tested positive for coronavirus - despite the fact that he'd had the virus in October.
Meanwhile others who have been in contact with positive cases have reported managing to escape a positive result.
Sir John Bell, regius professor of medicine at Oxford University, said some people may have already had a "background level" of protection against coronavirus.
He told The Sun : "I think they are present in quite a significant number of people.
"So there is probably background T-cell immunity in people before they see the coronavirus, and that may be relevant that many people get a pretty asymptomatic disease.
"Those T-cells get a bit tired once you're beyond the age of 65 and may not be as effective at removing a virus, so that may explain a number of different features of the disease."
Researchers in the UK and Brazil are reportedly studying whether it is possible some people might have a natural immunity which allows their immune systems to fight the virus.
A professor of medicine and immunology at Imperial College London, Danny Altmann, says the genes that control our immunity differ hugely from one person to another.
Referring to immunity, he told The Telegraph: "I’m talking about thousands of possibilities on your chequerboard; no two people will ever look the same."
One theory as to why some people may have natural immunity is that people could have been exposed to similar viruses before in the past, reports said.
When T-cells learn how to fight a similar virus, it is believed they become better at fighting others.
Some scientists suspect some people may just be born with immunity to certain viruses but this idea is less well researched and there is no clear evidence for this yet.
Labour leader Sir Keir Starmer tested positive for coronavirus a day after a major speech setting out his vision for Britain's future.
Sir Keir missed Prime Minister's Questions, with deputy leader Angela Rayner stepping in to face Boris Johnson.
The Labour leader previously tested positive for coronavirus on the day of the Budget in October.
Sir Keir is not thought to have any Covid-19 symptoms but the infection was picked up as part of his regular testing routine.
This latest positive test will mean it is the sixth period of self-isolation for the Labour leader since the start of the pandemic.
A Labour spokesman said Sir Keir had been taking regular lateral flow tests.
