Early research from China suggests women and children are less likely to die than men if they catch the coronavirus.
Death rates for Covid-19, the disease those infected with the coronavirus develop, are low for everyone: only 2.4 per cent of the 44,672 people in the Chinese study died.
But although roughly even numbers of men and women catch the disease, men are more likely to develop such a serious case of Covid-19 they die.
The death rate among the men in the Chinese sample - which covers late 2019 when the virus first emerged in the city of Wuhan up to 11 February - was 2.8 per cent, compared to 1.7 per cent for the women.
Children are even less likely to succumb to the coronavirus: no one under the age of 10 in the sample died, while just 0.2 per cent of those between 10 and 40 died.
Experts said it was unclear why Covid-19 seemed to affect men and women differently, although it was not the only virus known to do this.
Professor Paul Hunter from the University of East Anglia told the BBC women had "intrinsically different immune responses" to men.
"Women are more likely to suffer from auto-immune diseases, and there is good evidence that women produce better antibodies to vaccines against flu," he said.
Some of the difference is also thought to be because men are more likely to smoke and damage their health in other ways, rendering them more susceptible to developing a fatal case of the coronavirus.
This is particularly the case in China, where 52 per cent of men smoke, compared with just 3 per cent of women.
Research from the University of Iowa on SARS, a similar virus to Covid-19 which also emerged in China, has found female mice are less susceptible than males.
But when their ovaries were removed or the female hormone oestrogen was blocked, that difference disappeared, suggesting intrinsic hormonal differences were at play.
Young people's relative protection from the worst of Covid-19 is more of a mystery, however.
Ordinarily, the young would join the elderly and those with underlying health issues in the most at-risk group, which is the case with normal flu for instance.
But the new coronavirus does not appear to stimulate children's immune system into overdrive, which is what ultimately kills.
Most of those who catch Covid-19 will simply experience a fever and cough and then recover.
But in a small number, the virus will trigger an over-reaction from the immune system, prompting severe inflammation.
This inflammation in the lungs can then cause respiratory failure, or it can spread to other organs such as the kidneys and intestines. Heavily inflamed organs cannot function as they should, leading to multiple-organ failure and eventually death.
But among the young, this process does not seem to be taking place. "There must be something this virus does that is not as readily stimulating the immune system in children, but what that is is unclear," Professor Nathalie McDermott, from King's College London, told the BBC.
"They don't seem to be mounting a disproportionate immune response and some seem to be asymptomatic."
Dr Andrew Freeman, from Cardiff University, said this was not entirely novel as scientists had observed children were less susceptible to some other viruses.
"This does occur with some other viral infections such as Epstein-Barr virus infection which causes glandular fever in young adults, and also hepatitis A. We still have a lot to learn about this virus," he told The Daily Telegraph.
However, Professor McDermott also cautioned the sample size of under-18s in the Chinese study was small, particularly for babies.