How can you keep yourself safe from Covid-19?
This is the question tens of millions around the world are asking themselves every day during the pandemic.
The most important answer is to follow the social-distancing guidelines - but there are dozens of more complex factors being urgently investigated by scientists.
A range of characteristics are under interrogation to see how they might interact with the coronavirus, including diet, smoking and ethnicity.
The role of vitamin D, sometimes called the sunshine vitamin' is one of them and has become an area of intense interest for researchers trying to understand why certain people are disproportionately affected by the disease.
But what is it, why is it important and do we all need to be taking supplements to keep us safe this winter?
Here's everything we know about it so far.
Vitamin D is a nutrient which regulates how much calcium and phosphate we have in the body, which are needed to keep bones, teeth and muscles healthy.
There are two natural ways to get vitamin D: through sunlight in the summer months coming into contact with your skin or through your diet.
It is relatively uncommon in the food we eat but can be found in oily fish, red meat, liver, egg yolk and fortified foods (like fat spread and breakfast cereals), according to the NHS.
The UK has one of the highest rates of vitamin D deficiency in Europe, partly due to out gloomy winter weather.
Before Covid-19 existed, the Government was already recommending the use of supplements in the winter months (especially for people who spend a lot of time indoors, including the elderly, and people with darker skin).
Some research indicates vitamin D plays an important role in regulating the immune system, although the exact nature of this isn't firmly established.
The immune system is crucial here, not just because of its role in fighting off the disease initially, but also because there is growing evidence that an auto-immune overreaction which harms the body called a cytokine storm could be behind some Covid-19 related deaths (especially where there are no obvious underlying health conditions).
And because sunshine is a major source of the nutrient, specific attention was given to it due to increased time spent indoors during lockdown.
Mortality rates have also been higher in groups known to have deficiency, including older people in care homes and people from a BAME background (darker skin is known to be less effective at absorbing vitamin D).
In short, the jury is out - but there has been a clear shift in recent weeks towards putting a greater emphasis on the role of vitamin D in public health messaging.
Several studies in recent months have been trying to establish a link between rates of Covid-19 infection or mortality and with levels of vitamin D in a population.
One interesting study compared mortality rates from Covid-19 with latitude of countries to see if there are higher death rates in countries which don't get much sunlight in the winter.
It found that there was a possible connection, although there are some notable outliers (for example, high death rates in sunny Spain and Italy and low death rates in Nordic countries).
The study concluded that, while a link between vitamin D and Covid-19 isn't certain yet, there is good evidence it regulates the immune system and that more research is "urgently needed".
The Royal Society has acknowledged the latitude link and recommended the Government ramp up messaging about vitamin D supplement. Experts acknowledged the link is not conclusive but said "several lines of evidence suggest a possible role".
The National Institute for Healthcare Excellence (NICE) have released a report saying the link is inconclusive but the Government should continue to promote supplement taking.
Adrian Martineau, of the Institute of Population Health Sciences at Queen Mary University of London, was quoted by The Lancet as saying: At best vitamin D deficiency will only be one of many factors involved in determining outcome of COVID-19, but it's a problem that could be corrected safely and cheaply; there is no downside to speak of, and good reason to think there might be a benefit.
The NHS reissued its advice on taking vitamin D supplements in April to reflect circumstances of the lockdown: "Consider taking 10 micrograms of vitamin D a day to keep your bones and muscles healthy.
"This is because you may not be getting enough vitamin D from sunlight if youre indoors most of the day.
"There have been some news reports about vitamin D reducing the risk of coronavirus. However, there is currently not enough evidence to support this. Do not buy more vitamin D than you need."
There is more specific advice, including for children, here.
There is some compelling evidence that vitamin D and higher mortality rates from Covid-19 are linked but it is a long way off being conclusively established.
The Government and NHS were already advising us to take supplements during the winter - 10mg for most people and never more than 100mg - especially for people at higher risk of deficiency.
We should all follow that advice, especially as the summer months draw to a close.
We'll may have a better grasp of the link with Covid-19 soon but we know one thing for certain: Social distancing is still the best way to stay safe.