There are a number of possible explanations for the second positive test of the woman, in her late 40s, a resident of Osaka who worked as a tourbus guide. She first tested positive for coronavirus in late January and was discharged from hospital on 1 February after recovering. She tested negative again on 6 February.

It is possible, say experts, that when the woman was released, she had not cleared the virus. But if so, that means it lingered dormant in her body longer than the 14-day quarantine period. She will have been in contact with more people than have been traced, which poses worrying questions about the length of time people should be isolated after a positive test.

Alternatively, she may have been wrongly diagnosed with Covid-19 the first time round. But nobody is ruling out the possibility of reinfection. Once the immune system has fought off viral or bacterial infections, it generally recognises them and can block them the next time they are encountered – but not always and the protection may not last.

There have been reports of a few cases of reinfection in China, but doctors will hope it occurs in just a very few individuals, if at all.

Show more

After more than 82,000 cases of Covid-19, the unknowns still outnumber the knowns. Although there is clearly human-to-human transmission, we don’t know whether that happens only through droplets from coughs or sneezes or whether there are other forms of transmission as well. There have been reports of airborne transmission in China, although the World Health Organization (WHO) says it is generally not happening.

However, the infection of large numbers of people onboard the Diamond Princess cruise liner, which did not end after people were told to remain in their cabins, still needs explanation. Prof David Heymann, of the London School of Hygiene and Tropical Medicine and an adviser to the WHO, said there could be faecal or oral transmission as well.

Sewage was implicated in the cluster of Sars (severe acute respiratory syndrome) cases in the Amoy Gardens apartment block, in Hong Kong, in March 2003, when more than 300 people were infected. Sars, also a coronavirus, spread through the building’s plumbing system.

Heymann says the transmissibility of the coronavirus is still uncertain. “It is not know how transmissible this is in the community,” he said. All we know for certain is that it can be passed among groups in a small room, such as in families and in the German seminar room where several attendees were infected.

“There have been some cases in China and other places where they have just popped up without the possibility of being able to trace back to a source,” he said. Investigations in Italy are looking for some sort of mass event that could have led to the clusters of cases found in northern cities.

While we know there is asymptomatic transmission from somebody with the new coronavirus who is not ill, we do not know how extensive that is. Some people, known as “super-spreaders”, are more efficient transmitters of viruses than others.

Quick Guide What are coronavirus symptoms and should I go to a doctor? What is Covid-19? Covid-19 is caused by a member of the coronavirus family that has never been encountered before. Like other coronaviruses, it has come from animals. The World Health Organization (WHO) has declared it a pandemic. What are the symptoms this coronavirus causes? According to the WHO, the most common symptoms of Covid-19 are fever, tiredness and a dry cough. Some patients may also have a runny nose, sore throat, nasal congestion and aches and pains or diarrhoea. Some people report losing their sense of taste and/or smell. About 80% of people who get Covid-19 experience a mild case – about as serious as a regular cold – and recover without needing any special treatment. About one in six people, the WHO says, become seriously ill. The elderly and people with underlying medical problems like high blood pressure, heart problems or diabetes, or chronic respiratory conditions, are at a greater risk of serious illness from Covid-19. In the UK, the National health Service (NHS) has identified the specific symptoms to look for as experiencing either: a high temperature - you feel hot to touch on your chest or back

a new continuous cough - this means you’ve started coughing repeatedly As this is viral pneumonia, antibiotics are of no use. The antiviral drugs we have against flu will not work, and there is currently no vaccine. Recovery depends on the strength of the immune system. Should I go to the doctor if I have a cough? Medical advice varies around the world - with many countries imposing travel bans and lockdowns to try and prevent the spread of the virus. In many place people are being told to stay at home rather than visit a doctor of hospital in person. Check with your local authorities. In the UK, NHS advice is that anyone with symptoms should stay at home for at least 7 days. If you live with other people, they should stay at home for at least 14 days, to avoid spreading the infection outside the home. Show Hide

The WHO says we still need more information about the severity of the disease. In China, where the vast majority of cases and deaths have so far occurred, we know that 81% of people have had only mild illness. Of the rest, 14% have severe disease, which may become pneumonia, and 5% have critical disease involving breathing problems and organ failure.

The death rate has been estimated at between 2 and 4% in Wuhan, where the epidemic began, but only 0.7% in the rest of China. But we still do not know how many people are not being counted in these statistics, because they suffer only a sore throat and do not go to hospital.

It is clear that those people with damaged or failing immune systems are most at risk. “Older people, and those with pre-existing medical conditions (such as high blood pressure, heart problems or diabetes) appear to be more vulnerable,” says the WHO. But we do not know why children, who are very susceptible to some diseases, do not seem to be much affected by Covid-19.

We do not know whether any of the antiviral drugs in existence will help people recover – some, including anti-HIV/Aids drugs, are being given to patients as part of trials in China. We are probably at least 18 months away from knowing whether there can be an effective vaccine against Covid-19.

And we don’t know where and how this all began. Scientists understand from the the genetic sequencing of the virus that it came from animals, as did the other problematic coronaviruses, Sars and Mers (Middle East respiratory syndrome). Because of a cluster of early cases linked to the Huanan seafood market in Wuhan, which sold and slaughtered live animals for food, most believe the likely source to be wild animals. The similarity of the virus to Sars suggests a bat origin, but there would have been an intermediary animal carrying the virus, which could have been civet cats, bamboo rats or – as one Chinese university has claimed – pangolins.