Despite extensive study by doctors and scientists across the world since the new coronavirus first appeared in China three months ago, much about the scourge is still unclear.
Here are five key questions that remain unanswered about the virus officially known as SARS-CoV-2 that leads to COVID-19.
One of the biggest mysteries is why the virus produces few or no symptoms in around 80 percent of people, according to the World Health Organization, yet in others it can lead to fatal pneumonia.
"Findings from February indicated that the clinical spectrum of this disease can be very heterogeneous," said Professor Leo Poon from the School of Public Health at Hong Kong University.
During the peak of the epidemic in China, he and a team from the University of Nanchang compared those with mild and severe forms of the illness.
Their study, published in the British journal The Lancet, found that those hardest hit were "significantly older" and that the concentration of the virus in the nose and throat was around 60 times higher than those lightly affected.
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The question remains whether this result can be explained by the subjects' less reactive immune systems because of their age or whether they were simply more exposed to the virus.
Research on the measles virus has shown that the gravity of the illness is related to the level of exposure. Experts do not yet know whether this is the same for COVID-19.
Coronavirus is known to spread through physical contact and through droplets dispersed when an infected person coughs or sneezes.
But can it also hang in the air like the virus that causes the seasonal flu? One US study published in the New England Journal of Medicine shows that the new coronavirus can survive in air particles in a laboratory for three hours.
But scientists do not know if the disease can still be transmitted at that stage.
"We don't know if the virus can hang around for a long time in the air or on inert surfaces," Professor Karine Lacombe, head of infectious diseases at Saint Antoine Hospital in Paris, told AFP.
"We know we can find the virus, but we do not know if the virus is infectious."
But a recent study by the Harvard Medical School in the United States warned that "changes in weather alone (with an increase of temperature and humidity as spring and summer months arrive in the northern hemisphere) will not necessarily lead to declines in COVID-19 cases without the implementation of extensive public health interventions."