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If you’re over the age of 70, are pregnant, or have underlying health conditions, then it’s been drummed into you for weeks that you are the most at risk from coronavirus.

But as the death toll across the globe mounts, more and more cases have been reported of young and healthy people succumbing fatally to what, for most, should be a mild illness. How can that happen?

What is a coronavirus?

A coronavirus is actually the general name for a family of viruses that includes Sars-CoV2, the specific virus responsible for current global pandemic known as Covid-19.

Others include the non-lethal common cold, sometimes fatal seasonal flu, and the far more deadly Middle East Respiratory Syndrome (Mers), which has a fatality rate of around 34%.

How do coronaviruses kill people?

All coronaviruses cause similar symptoms – but their severity varies, which accounts for how fatal they can be.

Professor Ravi Gupta, of the Cambridge Institute for Therapeutic Immunology and Infectious Disease, told HuffPost UK: ”The feature they share is respiratory distress syndrome, which is characterised by the lungs filling up with inflamed cells.”

In mild cases, this inflammation can cause a cough and a sore throat, but in severe cases it can escalate, “essentially stopping oxygen from reaching your blood”, Professor Gupta adds.

Even in non-fatal cases, this can still be extremely unpleasant.

At the beginning of March, 29-year-old Daryl Doblados was well enough to run the Cambridge Half Marathon. Just two weeks later he was being rushed to hospital because his lungs felt like they were “filling up with smoke or liquid”.

In a video posted on Facebook, he said: “I struggled to breathe. I’ve never experienced anything like this before.”

What do we actually know about Sars-CoV2?

Very little compared to other coronaviruses. simply because it’s only been around for a few months.

Because of this and also due to the varying ways cases in different countries are recorded, the fatality rate for Covid-19 is still being worked out by scientists. Current estimates, though, put it at around 1.3% of cases. 

Is this the same for everyone?

No. As the government has warned, certain groups like the over 70s, people with compromised immune systems, and those with underlying health conditions are more at risk of developing a severe or fatal case.

For example, the latest data from the Office for National Statistics show of those who have died:

• 36% were aged 85 or over
• 33% were aged 75 to 84
• 19% were aged 65 to 74
• 11% were aged 45 to 64
• Just 1% were aged 15 to 44

Are older people always more at risk?

No. During the so-called Spanish Flu outbreak, up to 50m people died between 1918 and 1919. In an almost a total reversal of what the world is seeing now, 99% of the deaths recorded in the US, for example, were aged under 65.

Nearly half of all fatal cases were young adults aged 20 to 40 – and to this day, no one is sure exactly why.

Adding to the tragedy, many were young men who had survived the trenches of the First World War and returned home only to die during the flu pandemic just a year or less later.

Do all the younger people dying of coronavirus have ‘underlying health conditions’?

The majority, yes, but as the death toll has increased, more cases of young and healthy people dying of Covid-19 have emerged.

On March 31, it was reported that 13-year-old Ismail Mohamed Abdulwahab from Brixton had died in hospital after testing for the disease.

And on April 2 it was revealed that, of 44 coronavirus patients without underlying health problems who had died, one was just 27.

So… why are they dying?

The simple (but frustrating) answer is that we just don’t know for certain – it’s just too early to tell.

One possible theory concerns something called a “cytokine storm” where a healthy patient is essentially killed by their own, overreactive immune system.

“For some reason these viruses are setting up uncontrolled inflammation,” says Professor Gupta. “The body’s immune response becomes the main problem in that it becomes over-inflammatory.

“That may be one of the reasons why young people sometimes become very sick and die – because they have very strong inflammatory responses to the virus.”

What causes this response in some people but not others?

Again, we just don’t know at this stage. “It may be due to genetics or other infections they’ve encountered in the past – we don’t really understand what’s causing this,” says Professor Gupta.

“I think by the end of the year we’ll have a lot more information about what’s going on.”

Will we have a treatment?

While the ultimate prize to get the world through the coronavirus pandemic is a vaccine, there are drugs being trialled that may help those suffering now.

There is currently no approved treatment for Covid-19 and companies and doctors have been hunting for a drug already approved and in production to help keep thousands more patients from dying than standard care alone.

Those that target the excess inflammation caused by cytokine storm could be promising, meaning a number of arthritis drugs are currently being trialled.