What we know about the 'devious' coronavirus so far
Will the novel coronavirus leave you short of breath or make you lose your sense of smell? And if you have had it once, can you be confident you are immune? So far, the research on COVID-19 has raised as many questions as it has answered.
The tiny pathogen, which is spiked with proteins that scientists think it uses to latch on to the host cells of its victims, is surprisingly slippery.
"This virus is a nasty piece of work," says Jean-Francois Delfraissy, a specialist helping France with its COVID-19 response.
Here is what we know so far:
Who is most at risk?
There is a growing wealth of data that suggests the severity of COVID-19, the disease caused by the SARS-CoV-2 virus, increases with age.
Recent research, largely focused on several hundred cases observed in China in late February, suggested the disease is on average much more dangerous for those aged over 60, a group that has a mortality rate of 6.4 percent among confirmed cases.
But this climbs higher the older patients are, with 13.4 percent of cases aged over 80 leading to death, against only 0.32 percent for those under 60.
The study, which was published in the British medical journal The Lancet on March 31, also said the proportion of people with COVID-19 needing hospital care increased sharply with age -- while only 0.04 percent of 10 to 19-year-olds required hospitalisation, that rose to 4.3 percent for those in their forties.
And almost one in five people in their eighties with the disease -- 18.4 percent -- developed a form serious enough to need treatment in hospital.
Chronic illnesses can also raise the risk.
In a report looking at 20,000 deaths, Italy's National Institute of Health (ISS) found the most common underlying health problem was hypertension, seen in 69.7 percent of cases, followed by diabetes in 32 percent, and coronary heart disease in 27.7 percent.
Finally, according to a large analysis published in February by Chinese researchers in the American medical journal JAMA, the disease is mild in 80.9 percent of cases, serious in 13.8 percent and critical in 4.7 percent.
How many deaths?
If you were to compare the number of deaths in the world to the total number of officially registered cases, you might conclude that COVID-19 kills roughly seven percent of diagnosed patients, depending on the country.
But this number should be taken with caution -- it is unclear how many people have been infected.
Many people seem to develop few or no symptoms and are for the most part not counted in the official figures, with countries taking markedly different approaches to testing and many not following up on every suspected case.
If official tallies included figures for milder cases, the mortality rate would likely be significantly lower.
With those undetected cases added "that probably brings the mortality rates down to somewhere around one percent," top US government scientist Anthony Fauci said in comments to Congress.
"Which means it is 10 times more lethal than the seasonal flu," he said. "This is a really serious problem that we have to take seriously."
The dangerousness of a disease partly depends on its ability to spread. Even a mortality rate of one percent can translate into a significant death toll if the disease affects a large proportion of the population.
Pressure on hospitals caused by a huge influx of patients has also exacerbated COVID-19's toll.
What are the symptoms?
The World Health Organization lists the most common symptoms as fever, fatigue and a dry cough. But it says some patients may experience a variety of other ailments, from a runny nose to diarrhoea.
The virus can often cause shortness of breath, which in the most severe cases can lead to respiratory failure. It is also becoming increasingly clear that the coronavirus can affect the brain and nervous system.
Field observations and several studies have described neurological symptoms: loss of smell and taste, nervous pain, confusion, even seizures and strokes. This could be due to a phenomenon known as a "cytokine storm", a rapid overreaction of the immune system.
There is no vaccine or medication, and management involves treating the symptoms. However, some patients have been given antiviral drugs or other experimental treatments, as scientists look for treatments.
How is it transmitted?
The virus is mainly transmitted by respiration and physical contact, and it seems that a patient can already be contagious even before the first symptoms appear.
Respiratory transmissions occur via droplets of saliva expelled by an infected person, for example in a cough. Scientists believe that this would require close contact, at a distance of about a metre (three feet).
You could also become infected by touching a contaminated object and then putting your hand to your face. Two studies, one published in mid-March and the other in mid-April, in the US New England Journal of Medicine found the new coronavirus is detectable for up to two or three days on plastic or stainless steel surfaces, and up to 24 hours on cardboard.
However, these maximum durations are only theoretical. "These studies have evaluated the presence of genetic material, and not of a live virus", French health authorities have said, adding that the ability of the virus to contaminate diminishes very quickly outside.
Another unknown is the ability of the virus to circulate in the air -- what scientists call an aerosol -- and to contaminate in this way. To avoid contagion, health authorities have emphasised barrier measures: avoid shaking hands and kissing, wash hands frequently, cough or sneeze in the crook of your elbow. And wear a mask if you are sick.
Can you be infected twice?
South Korea has recorded cases of patients who apparently recovered and tested negative, then tested positive again, raising questions about whether you could catch the virus twice.
The most commonly accepted assumption is that these patients never really recovered. Their negative result could be explained either by a very low presence of the virus in the body, or by the fact that the test had been poorly performed. But the extent of immunity for those people who have had the disease is still far from certain.
French coronavirus adviser Delfraissy said the virus was "devious", telling a recent parliamentary committee that some evidence suggested the possibility of "reactivation". He wondered "if we are not completely mistaken" in relying on the idea of immunity.
But if that were the case it would be extremely difficult to control the epidemic because people could become repeatedly infected.