As scientists continue to rip Covid-19 apart, approaching it from every angle imaginable, mysteries persist. Science is full of mysteries, which is what keeps scientists going, trying to provide answers. The biggest question when it comes to Covid-19 is obvious: what's going to happen next? When will things go back to normal? Scientists are trying hard to get an answer to that one. Slowly things are returning to normal in China but not fully, with many restrictions still in place. China is being watched closely. Scientists are also watching Africa, since if it takes off there it will be a huge humanitarian disaster and the virus will be there for a long time - a reservoir to keep reinfecting the rest of the world. A second surge later in the year remains a possibility, with more lockdowns, which nobody wants.
Robert Redfield, the head of the Centre for Disease Control in the US (which has the best scientists when it comes to infectious diseases) has stated some things we definitely know.
First, this is not like the flu: it is three times as infectious and kills at around 10 times the rate of flu. At least 25pc of infected people have no symptoms and can infect others. Those who become symptomatic are infectious four to eight hours before symptoms begin, making infection very hard to contain. Social distancing is the most powerful weapon we currently have. If done properly, this will shut this outbreak down. We also know that most of us (99pc) who get Covid-19 will recover. A vaccine will ultimately be the thing that will protect us and allow life to return to normal although in a different world.
Given all of this, three interesting things are puzzling scientists and they're not all bad. Getting answers might help us.
Why are children largely unaffected?
Firstly, and with huge relief, children are largely unaffected by Covid-19, as was the case with the original SARS outbreak in 2003. Children and babies are not dying of Covid-19, which was not the case with the 1918 flu pandemic where children under two and pregnant women were especially vulnerable. Covid-19 is effectively an older person's disease since most people under 60 have a mild course. Tests in mice support the notion that it is more deadly in the old. Using the related virus that causes SARS, one-fifth of mice infected with SARS aged 3-4 weeks died, whereas all of the mice aged 7-8 weeks died.
In spite of this, children do get infected and can infect older people, so as ever, they must practise social distancing like everyone else, difficult and all as that is.
So why is it that children are protected? One explanation for the correlation between age and disease severity is that, like everything else in your body, your immune systems age too and become less agile. When you're young, it may be that you have much more of a virus-busting immune cell called a T cell. These die off as you age, allowing the virus to get a foothold in older people.
T cells are very clever: they can also put a brake on inflammation, which is a powerful response that can help fight infection. The only trouble is, inflammation can sometimes overdo it, and the T cells can wag a finger and say: behave. Less T cells as you age might mean that inflammation runs riot, damaging your lungs, which can be lethal.
A final possible reason why children are protected is some of the other corona viruses. Four of these cause the common cold and children have more colds. This might provide what's called cross-protection against Covid-19, although we don't know. Finding these things out will help the effort to make a vaccine, because if you can get the right T cells to respond in older people, or use parts of other corona viruses, you might get a really effective vaccine.
Why doesn't everyone get sick?
Mystery number two is why is it that when one person is sick in a family or household, not everyone gets sick? All over the world this is being noticed. Someone has it, and then someone else in the house gets it but not another. The one who escapes it feels blessed but what does science tell us?
Reason number one is the amount of virus being spread. There is good evidence that getting infected is directly related to dose of virus. This makes sense. If you touch a door knob with 10 viruses you are a lot less likely to become infected than if there are 10,000 viruses. So maybe those who catch it were exposed to more viruses, hence the importance of keeping to the hygiene rules.
Then there's the question of how strong your immune system is. This varies between people, because of diets, level of fatigue, stress and also overall fitness. Maybe someone had a bad night's sleep and their immune system was tired when they got exposed.
Finally, genetics play a part. Just like we all have different faces (unless we're identical twins), we all have slightly different immune systems. Scientists are looking hard at differences in immune genes in patients to try to find out who might be more susceptible and who might get really sick. A gene variant called HLA-B*46:01 has been suggested to increase vulnerability. This could be tested in people, although it's early days for that kind of thing. But genetics has to be part of the picture - maybe the virus likes your 'immune face' but not your brother's, and so you get infected and he doesn't, jammy beggar.
More seriously, genetics may help us hugely in predicting who might have a more severe disease, which will help with caring for that patient.
Why are men hit worse than women?
The third mystery is why are men suffering more than women when it comes to Covid-19? Women definitely have the upper hand when it comes to this virus. In Italy, the mortality in males is twice that of females in every age group. Overall, around 70pc of those who have died are men.
Scientists say there are several factors that might be responsible for this. In China, one reason might be smoking, which makes the lungs more vulnerable. There, 50pc of men smoke, whereas only 3pc of women do. Seven million Italian men smoke. One in four people ending up in intensive care are smokers. Smokers may also be at a higher risk of infection because they touch their mouths more. Men are also less likely to wash their hands than women.
Another reason might be underlying diseases. More men than women have high blood pressure, heart disease, lung diseases like COPD, and diabetes when compared to women - all risk factors for death from Covid-19.
Another factor might be hormones. In mice the female hormone estrogen has been shown to protect against SARS.
Finally, it turns out that the X chromosome, which women have two copies of, harbours a lot of immune genes. Women might have a double dose of virus killers in their bodies and so are protected more.
Questions, questions, questions. But answers are coming, and these answers all help in our efforts to beat Covid-19, and to prepare us for the next virus that might come along.
Luke O'Neill is professor of Biochemistry in the School of Biochemistry and Immunology at Trinity College Dublin