US President Donald Trump at Cleveland Hopkins International Airport in Cleveland, Ohio, 29 September 2020

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Donald Trump has clear risk factors – including his age, weight and being male – that all raise the chances of a severe coronavirus infection.

He is 74 and has a Body Mass Index (BMI) over 30, which is the clinical definition of obesity.

So now he has tested positive for the virus, what does it mean?

At this point, Mr Trump has only “mild symptoms” and is being treated with drugs that slow the virus’s assault on the body.

But age is a clear and strong link to developing a severe infection, leading to hospital treatment and in some cases death.

“But most people who get an infection also get better,” Dr Bharat Pankhania, from the University of Exeter medical school told the BBC.

An early analysis of more than 100 studies, taking in data from around the world, showed the risk for children and young adults was tiny.

However, at age 75 it is estimated that one in 25 people who catch coronavirus dies. This rises to one in seven people aged over 85 and one in four over the age of 90.

A similar pattern has been seen by the US Centers for Disease Control (CDC).

It says eight out of 10 Covid deaths in the country have been in the over-65s. And people in Mr Trump’s age group are five times more likely to need hospital care and 90 times more likely to die than somebody in their 20s.

It is not clear if the reason is something fundamental about being older, such as the immune system becoming less efficient with time, or if our age is just shorthand for being in generally worse health.

“Age is also tied to heart disease, lung disease, type 2 diabetes, so it’s hard to disentangle whether it is their age or comorbidities (other health problems),” Prof Ravindra Gupta, from the University of Cambridge, said.

Obesity has also been linked to more severe disease.

A review of the evidence by Public Health England concluded excess weight increased the chances of needing intensive care treatment and the possibility of death.

Fat in the body can influence the white blood cells of the immune system to increase levels of inflammation in the body. Excessive inflammation is why the infection can become deadly.

But again, obesity is linked to a host of underlying health problems including type 2 diabetes and heart disease.

In the first wave of the pandemic there were more men than women needing hospital treatment with 60% of Covid-19 deaths occurring in men in the UK.

“There are immune differences between men and women,” said Prof Ravindra Gupta, which might explain the difference.

But again, men tend to be in worse health to begin with.

Prof Pankhania added: “We do know that being male and older may lead to more severe illness, but that doesn’t translate into every older man getting severe illness.”

The problem is that it is dangerous to speculate about how the virus will affect any individual person.

Everything you have read is the big picture – a pattern seen across the whole population – it is not a way of saying this is what will happen to the US president.

The health of any two 74-year-olds can be wildly different and other medical conditions massively alter how big a threat this virus poses.

So far, Donald Trump has only developed mild symptoms, according to White House officials.

“Just because he has risk factors does not mean he is going to have severe disease or succumb to the illness,” says Dr Nathalie McDermott, from Kings College London.

She added: “He’s also the president of the United States, so he has the best healthcare in the world should he become unwell.”

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