When it comes to Donald Trump's risk of becoming seriously ill with coronavirus, there are "known knowns", and there are "known unknowns".
The US President is characteristically hyperbolic about his health, insisting he is "blessed with great genes" and "superb" mental abilities.
"I consider my health, stamina and strength one of my greatest assets," he has previously stated.
A letter from his personal doctor, Harold Bornstein, in 2015 described the presidential candidate's health as "astonishingly excellent" adding that "Mr Trump will be the healthiest individual ever elected to the presidency".
It later emerged that Trump had dictated the report himself.
What is clear is that the president has several factors stacked against him including his age, weight and sex.
When all his details were entered into the US "19 and Me" coronavirus risk calculator, the president was found to have a 35 per cent risk of hospitalisation, an 11 per cent risk of needing intensive care and an 8 per cent risk of death.
Why the virus could pose a real danger to the president
More than 200,000 people have now died in the United States, and undoubtedly the biggest risk factor is age.
At 74, Trump is the oldest first-term president in US history and falls into an age bracket (65 to 74) which has recorded 21 per cent of all US deaths.
The chance of death increases about 12 to 13 per cent with each year, and doubles every five to six years.
It means a 20-year age gap increases the risk by around 10-fold, which means compared to a 20-year-old, an 80-year-old has 1000 times more risk of death.
The US Centres for Disease Control and Prevention (CDC) reports that people aged 65 to 74 have a five times higher risk of hospital admission with coronavirus compared with those aged 18 to 29, and a 90 per cent higher risk of death.
For people aged 75 to 84, the risk of needing hospital admission jumps to eight times higher, with a 220 times higher risk of death.
Michael Head, senior research fellow in global health at the University of Southampton, said: "The president's profile would classify him as vulnerable. He is aged 74, and reportedly overweight. Many people in their 70s will also have further co-morbidities that increase the risks of a more severe illness."
Sex and ethnicity
It also does not help Trump that he is a man. Overall, men are more likely to die from Covid-19 than women. The latest figures from the CDC show so far in the US, about 104,896 men have died from coronavirus, compared to 89,191 women.
However, being white is less dangerous than being from a Black, Asian or Minority ethnic background.
People of Indian, Pakistani and Bangladeshi heritage are 20 per cent more likely to die if they catch coronavirus. Although Black people are disproportionately impacted because they are more likely to catch the virus in the first palace and be hospitalised, they are no more likely to die from it.
Obesity is also a major risk factor for severe Covid.
The president weighs in at 17 stone 6lbs (110.7kg) and is 6ft 3in tall, which means he is clinically obese and has a Body Mass Index (BMI) of 30.3. The NHS recommends someone of Trump's height and build should lose 12lbs to be healthy.
A review of studies by Public Health England (PHE) found a dramatic rise in the risk of hospital admission and death from Covid-19 for people who are overweight or obese.
One study analysed found that, compared with a normal BMI, being overweight increased the risk of admission to intensive care by about 50 per cent, rising to more than double the risk for those with a BMI of 30 to 35, and about a four-fold increased risk for those with a BMI over 35.
A separate study found that, for people with a BMI of 35 to 40, risk of death increases by 40 per cent, and with a BMI over 40 by 90 per cent.
However, the president is a keen golfer and appears to walk quickly, which experts say is an indicator of good health. He is also known to only eat the toppings of his pizza for health reasons.
Stephanie Grisham, White House press secretary has previously insisted: "He's got more energy than anybody in the White House. That man works from 6am until, you know, very, very late at night."
Prof Naveed Sattar, Professor of Metabolic Medicine, University of Glasgow, said "Donald Trump has some risk factors such as being male, older (still strongest risk factor) and overweight but if he has no chronic conditions and is reasonably active (it seems likely so as he plays lots of golf and appears to walk quickly) then these may offset or attenuate his risks so that he may recover swiftly from the infection.
"The risk of serious disease and death depends on many factors – some unmeasurable, so there is always uncertainty - and it is not so simple as to make inferences from one or two alone."
Although the president claims to have no underlying conditions, in February 2018, medical records released by the White House suggest that Trump has a common form of heart disease.
Trump underwent a coronary calcium CT scan as part of his routine physical exam and his score was 133. Anything over 100 indicates plaque is present and the patient has heart disease.
At the time, doctors said he might experience a heart attack in three to five years, but the problem could be addressed with cholesterol-lowering drugs like statins, or lifestyle changes. It is not known if Trump takes any medication.
The president does not drink or smoke, which are also risk factors regarding Covid.
There are also concerns the president could be suffering from a neurological condition after several incidences where he has appeared unsteady on his feet and his speech has appeared slurred.
On June 13, the president explained on Twitter why he was seen walking falteringly down a ramp at the US Military Academy's commencement ceremony watching his feet the whole time. He claimed the ramp was steep and slippy, but specialists said it looked like he was having trouble with his balance, and the #TrumpIsNotWell hashtag has continued to trend on social media.
Dr James Merikangas, a neurologist and clinical professor of neuropsychiatry at George Washington University School of Medicine, said he long noticed issues with how Trump carries himself during public appearances and believes he has a neurological problem.
Heart disease and neurological conditions are known to be a risk factor for severe Covid. Based on early US reports, 40 per cent of hospitalised Covid-19 patients had cardiovascular disease or cerebrovascular disease.
If the president does have heart disease, his risk of being hospitalised from Covid would rise to 46 per cent, while his chance of needing intensive care would rise to 16 per cent. He would also have a one in 10 risk of death.
However, even if the president makes a full recovery, large numbers of people are now suffering the effects of "Long Covid".
In September, researchers at North Bristol NHS Trust found that three quarters of virus patients treated at Bristol's Southmead Hospital were still experiencing problems three months later.
Symptoms included breathlessness, excessive fatigue and muscle aches, leaving people struggling to wash, dress and return to work.
What about Melania?
Melania is younger, has a healthier weight and a female, so has far less risk of becoming seriously ill.
The "19 and Me" calculator estimates Melania has a 5 per cent risk of hospitalisation, just 1.3 per cent risk of needing intensive care treatment, and a risk of dying of 0.4 per cent.