A Japanese man who appeared to have recovered from coronavirus tested positive again less than three weeks after he left a medical facility where he was being treated.

The man, in his seventies, was a passenger on the Diamond Princess cruise ship and first tested positive for the virus on 14 February while onboard the then-quarantined vessel.

He was put in further quarantine and treated at the medical facility in Tokyo and tested negative on 2 March, reported Japanese news agency NHK.

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But after returning to his home in the Mie Prefecture, within the Kansai region, he began exhibiting symptoms of the virus again and developed a fever of 39C on Thursday.

The following day, he went to a hospital to be retested, and reportedly was confirmed to be infected again on Saturday.

His is not the only case of apparent coronavirus reinfection. On 27 February, a Japanese woman tested positive for the virus for a second time after seemingly recovering from the illness several weeks before.

The woman, an Osaka resident in her forties, was believed to be the first person to have tested positive for Covid-19 a second time outside of China.

Earlier this month, doctors from different parts of China reported some patients who recovered from the illness and were discharged had tested positive for a second time.

The incidents prompted Dr Zhang Zhan, from the Renmin Hospital of Wuhan University’s Department of Respiratory and Critical Care, to call for stricter patient discharge requirements.

As a new virus, there is still much experts don’t know about how Covid-19 behaves. Some experts said it could be bi-phasic, meaning the disease could appear to go away before recurring. Certain individuals, including those who are immunocompromised, may also be at higher risk of contracting the virus again, a hospital director in Beijing, Li Qingyuan, told USAToday.

Dr Edward Wright, a virologist at Sussex University's School of Life Sciences, told The Independent: "There's still a lot we don't know about this virus and there could be several reasons why a person could test positive, then negative, then positive again. It could have been a mistake in the diagnostic test, or there are some examples of viruses becoming persistent - coronaviruses are not known to become persistent but we just don't know yet.

"Getting reinfected is unlikely, but it's also not something that can be ruled out."

The Centres for Disease Control and Prevention (CDC) explained: “The immune response to Covid-19 is not yet understood. Patients with Mers-CoV infection are unlikely to be reinfected shortly after they recover, but it is not yet know whether similar immune protection will be observed for patients with Covid-19.”

Leading scientists also said there is no guarantee of people becoming permanently immune to the virus after being infected and recovering.

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Responding to the UK government’s policy of allowing large swathes of the British population to fall ill with the virus in hopes of creating “herd immunity”, officials warned it is too soon to know if people will become permanently immune to Covid-19.

Dr Nicola Rose, head of virology at the National Institute for Biological Standards and Control, said: “We don’t know how the pandemic will evolve. With flu, for example, it comes and goes in waves. This may well be the case here. We really don’t know whether there will be herd immunity or not.”

People who have been infected should follow hygiene steps outlined by CDC and other health authorities, said Mr Wright, including keeping a distance away from people who are sick, washing hands often with soap and water, and covering coughs and sneezes.

"At the moment, people really need to think about if they really need to make that trip, or what they can do to limit the risk of passing the virus on to someone else who could seriously be affected by it," he said.

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