paris 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 unclearhere are five key questions that remain unanswered about the virus officially known as sarscov2 that leads to covid19one 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 pneumoniafindings 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 universityduring 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 illnesstheir 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 affectedthe 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 virusresearch 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 covid19coronavirus is known to spread through physical contact and through droplets dispersed when an infected person coughs or sneezesbut 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 hoursbut scientists do not know if the disease can still be transmitted at that stagewe dont 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 afpwe know we can find the virus but we do not know if the virus is infectiousapart from the few countries that have used aggressive testing like south korea and germany global figures on the number of people carrying the virus are sketchythe british government estimated that on march 17 55000 people there had the virus when less than 2000 had officially tested positivea precise understanding of the spread of the epidemic is vital to isolating and treating those infected as well as developing a profile of those who can build up immunity to itthis will be possible once a new generation of blood tests are developed that are capable of tracing the immune reaction to sarscov2will covid19 be halted with the arrival of warm spring and summer weather in the northern hemisphere experts say this is possible but not certainrespiratory viruses like the flu are more stable in cold and dry weather which is why they spread more rapidly in