The new coronavirus is making headlines all the world, despite being virtually unheard of just two months ago.
Since emerging at a seafood and live animal market in the Chinese city Wuhan at the end of last year, the Covid-19 strain has spread thousands of miles from its epicentre, crossing borders into more than 30 countries.
The figures paint a bleak picture, with more than 83,000 confirmed cases on Friday, of which over 78,000 were in mainland China, according to John Hopkins University data.
While cases appear to be plateauing in China, South Korea has more than 2,300 patients, of whom 13 have died.
Italy is the most affected country in Europe, with over 650 cases and 17 fatalities.
The UK has 19 confirmed incidences.
Globally, the death toll is approaching 3,000.
Perhaps reassuringly, the number who have beaten the infection is also on the rise, with 36,700 confirmed “recoveries”.
While that may sound a positive step in controlling Covid-19, fears flared after a woman reportedly became “reinfected” with the virus.
How likely is it patients will become reinfected with coronavirus Covid-19?
The unnamed woman, from Osaka in western Japan, tested positive for the virus on 29 January after working as guide for tourists from Wuhan.
The woman – who is in her forties – was discharged from hospital on 6 February despite still having a cough, a symptom of Covid-19.
After feeling well for a week, she went back to her doctor on 21 February complaining of a sore throat and chest pains, only to be diagnosed again.
This phenomenon has also been reported in China.
Statistics from hospitals in Guangdong province suggest around 14% of patients who “recover” from Covid-19 later test positive for it, the Daily Star reported.
Not all experts are convinced, however.
Speaking of the Japanese woman, Professor Paul Hunter – from the University of East Anglia – said: “There is so much we do not know about this case to give a properly informed opinion.
“Did the woman test negative after her last positive and if so how many tests were negative before her initial discharge?
“Does she have any underlying illness or is she on any treatment that could affect her immune system?”
He noted fellow coronavirus severe acute respiratory syndrome (Sars) – which killed 774 people during its 2004 outbreak – “relapsed” in one patient who was treated with steroids, an immune-suppressing drug.
With Covid-19 seemingly arriving out of nowhere, little is known about how it is transmitted, how infectious it is or how many could die.
“I’m not certain this is not biphasic, like anthrax”, Professor Philip Tierno Jr, from New York University, told Reuters.
Around four days after infection with the bacteria Bacillus anthracis, anthrax typically triggers flu-like symptoms, also lasting four days.
Without treatment, a second “phase” of low blood pressure and breathlessness sets in, which can kill within 24 hours.
Professor Hunter said it is “possible” Covid-19 is biphasic but “unlikely to be common based on current information”.
He questioned whether the woman had “prolonged excretion of the virus from her initial infection”.
She may also have been given the initial “all clear” without tests or based on tests not “done sufficiently well or enough to confirm clearance”.
“I would caution against reading too much into this report given the lack of information,” said Professor Hunter.
“However, the report reinforces the fact we have to investigate all such findings really thoroughly and report such information if we are to make the best decisions based on sound scientific evidence.”
Other experts were equally sceptical.
Professor Mark Harris – from the University of Leeds – called the case a “clear concern”, but added it is “unlikely” the woman was reinfected.
“Most [patients would] likely have mounted an immune response to the virus that would prevent such reinfection”, he said.
“The other possibility is they did not in fact clear the infection but remained persistently infected”.
Professor Rowland Kao, from the University of Edinburgh, added: “It remains unclear whether the person involved was likely reinfected, or whether this represents an infection that may have been partially cleared or perhaps has gone latent”.
A latent virus “lies dormant”, without multiplying or typically causing symptoms until it is “reactivated”. An example is the herpes virus that triggers cold sores.
“Given the number of reported cases thus far, it would seem unlikely this is a common occurrence and thus should have only a small impact on the overall epidemic projections themselves”, Professor Kao said.
What is the coronavirus Covid-19?
Covid-19 is one of seven strains of the coronavirus class that are known to infect humans.
Others range from the mild common cold to Middle Eastern respiratory syndrome (Mers), which killed 858 during its 2012 outbreak.
Four out of five cases are mild, with patients developing flu-like fever and breathlessness.
In severe cases, pneumonia can come about when the infection causes the alveoli (air sacs) in the lungs to become inflamed and filled with fluid or pus.
The lungs then struggle to draw in air, resulting in reduced oxygen in the bloodstream.
“Without treatment the end is inevitable,” said the charity Médecins Sans Frontières.
“Deaths occurs because of asphyxiation.”
While no one can say for sure where the virus came from, bats seem most likely.
The nocturnal creatures are thought to have been behind Sars and Mers.
Scientists from Peking University in Beijing suggested snakes may have been the “intermediate host” for Covid-19.
A team from South China Agricultural University later found it could have “jumped” from bats to humans via pangolins.
Covid-19 has no specific treatment, with care being “supportive” while a patient’s immune system works to fight off the virus.
To prevent infection, the NHS recommends regular hand-washing and avoiding those with suspect symptoms.