The odds were stacked against 100-year-old Elsie Kelsall.
Three weeks ago, the great great-grandmother from Staffordshire was taken to hospital with a high temperature and mild cough - both symptoms of Covid-19. Two days later, her family's worst fears were confirmed when tests came back positive.
Her fate, they thought, was sealed. About 90 per cent of those who die from this disease are over 65. But, amazingly, after just 12 days of treatment, Elsie was discharged. Even more remarkably, her condition remained stable throughout and there was no need for significant medical intervention, let alone a stint in intensive care.
As Elsie left Haywood Hospital in Stoke-on-Trent in a wheelchair and with a blanket around her knees, dozens of NHS staff formed a guard of honour to clap and cheer as she passed by. Now, living at home with the support of carers, Elsie is tired but well on the road to recovery
'My mother is a survivor,' says her 76-year-old son, Malcolm Kelsall, who runs an engineering business. 'It seems as if she has flown through the illness. She had a high temperature and a little bit of a cough, but nothing severe. It's amazing really - with some people it knocks them for six.'
Amid a barrage of bad news and gloomy headlines, more heartwarming stories have emerged of centenarians who have defied the odds.
At 106, Connie Titchen, from Birmingham, is the oldest known person in Britain to have survived Covid-19. The great-grandmother-of-eight, who has lived through both world wars and the Spanish flu pandemic which killed millions, was admitted to hospital with suspected pneumonia in mid-March. She tested positive for Covid-19, but after just three weeks she had made a full recovery.
Keith Watson, 101, from Worcestershire, also overcame the illness after a fall at his care home landed him in hospital. And, in the Netherlands, 107-year-old Cornelia Ras is believed to be the oldest in the world to have survived the virus. She suffered only very minor symptoms.
It's certainly a curious pattern. All the evidence suggests the biggest risk factor for death by Covid-19 is age. Yet some emerge perfectly healthy. So just how has this select group - dubbed the super-survivors - pulled through?
'It's wonderful that some people have beaten the odds of their age group,' says Sarah Harper, professor of gerontology at Oxford University. 'But it was not unexpected that we would have survivors of every age. Some people in their 80s, 90s and 100s have a physical or psychological advantage over the majority of those their age. This can be due to genetics, their environment, lifestyle - or sometimes just luck.'
The answer - among those aged beyond 100, at least - may lie in having a genetically superior immune system.
'We can speculate that these special older, and very old people, have a particular genetic makeup that contributes to a healthy immune system,' says Prof Harper. 'It's feasible that the way their immune system is responding to Covid-19 is much more similar to that of a younger person.'
According to the latest figures from the Office for National Statistics, more than a third of Covid-19 deaths have occurred in people over the age of 85.
'In general, our immune systems become slower and less effective at fighting off infections as we age,' says Professor Arne Akbar, president of the British Society for Immunology. 'This applies to viruses our body has seen before, like chickenpox, and newer ones, such as the coronavirus.'
In younger people, when new pathogens or viruses enter the body, the immune system releases white blood cells to find and destroy them. But in older people, the immune system is less able to recognise threats. Even if it does respond, the white blood cells are less effective at stoping the disease from taking hold.
Another reason why older people bear the brunt of coronavirus is that they are much more likely to have underlying health conditions. Diabetes and heart disease, for instance, make patients far more vulnerable to severe illness caused by Covid-19.
Even so, some older people have proved exceptions to the rule. Early in the outbreak, Health Minister Nadine Dorries contracted Covid-19 and passed it on to her 84-yearold mother, who suffers a heart condition. Both recovered, but it was Nadine, despite being more than 20 years younger, who suffered most.
As Prof Akbar explains that when it comes to ageing, everyone is different.
'Some older people are still out there cycling, jogging and running, whereas others who are younger, in their 50s even, are almost bed-ridden,' he says. 'Not everyone ages at the same rate - and there is not a fixed point where you start going downhill. Some people start that process earlier and some people later.'
However, when it comes to the very elderly, it's much more likely that the secret to surviving Covid19 lies with their genetics.
'There's something in their genes that makes them more resilient than other people,' says Prof Akbar. 'It means they are more likely to be able to fight off an infection such as this.'
According to the latest estimates, there are about 13,000 centenarians in the UK, of whom 800 are aged over 105.
'The over-105-year-olds tend to be much, much healthier - not only than their own cohort, but than people who are ten or even 20 years younger,' says Prof Harper. 'They seem to have very little cancer, very little cardiac disease, very low levels of diabetes, and we think they may have better physical and cognitive function generally too.'
Dozens of studies support this. One published in the Journal Of Gerontology in 2017 compared the final six years of life in people over the age of 100 with others who died in their 80s or 90s. The German researchers found that, on average, the centenarians suffered from fewer diseases throughout life than their younger counterparts. The increase in the number of conditions normally experienced in the last few years of life was also less marked in centenarians.
Coronavirus survivor Elsie Kelsall is one such example, having lived for most of her 100 years without any underlying health problems.
'Her short-term memory is not what it used to be, but she remembers things from 50, 60, 70 years ago no problem,' says her son Malcolm
Separate research in Okinawa, Japan - a country that's known for the longevity of its inhabitants - examined the health of 12 people aged 110 or over. The researchers found they had little history of cardiovascular disease and no cancer or diabetes. The onset of chronic diseases and disabilities was also significantly delayed, often to beyond their 100th birthday.
Experts believe this evidence supports the idea that some people have a so-called 'healthy ageing phenotype' - a set of special genetic and lifestyle characteristics which helps them evade illness and disease, even in old age. But, as yet, scientists don't fully understand it or why it occurs in the people it does. And they are yet to isolate a specific gene responsible for these extraordinary traits.
Yet some suggest that the 'healthy ageing phenotype' could help support an unusually robust immune system.
'We now understand that just as our immune system fights infectious diseases, it is also involved in fighting chronic diseases such as cancer,' says Prof Harper. 'It may be that the immune systems of these very special older adults are also able to fight off an acute infection like Covid-19.'
However, genetics do not explain the whole picture. Last month, Dr Hans Kluge, the World Health Organisation's regional director for Europe, stressed the importance of baseline fitness for overcoming the lockdown virus for all ages.
'It is becoming clearer that the healthier you were before the pandemic plays a crucial role,' Dr Kluge said. 'People who age healthily are less at risk.'
It is an idea echoed by Sir Muir Gray, professor of primary healthcare at Oxford University and director of the Optimal Ageing Programme.
'People who are 90 or 100 vary in more ways than they are similar,' he says. 'So you have always got to be cautious when looking at chronological age alone. There are four main factors which affect our health as we live longer - and each of these could have a part to play, whether we are dealing with cancer, heart disease or Covid-19.'
The first, he says, is the normal, biological process of ageing, which is influenced by genetics. The second is loss of fitness.
'In studies of people over the age of 100, physical activity is one of the main things that seems to mark them out from the rest,' he says.
A study of more than 300 Dutch centenarians, published in the European Journal Of Epidemiology, found that almost half needed just a minimum amount of help with daily tasks, while a third were fully independent. Elsie has always been active, working in a factory well into her 60s, according to her son.
'She's a bit frail now, but up until her 90s I used to walk with her to fetch a newspaper,' he says. 'In the past three or four years she's got a bit unsteady on her feet, but she still walks up and down the house.'
The third factor is illnesses such as lung and heart disease - often caused by lifestyle factors such as smoking and poor diet, which may be preventable.
The fourth, Sir Muir says, is belief and attitudes, with research suggesting that maintaining a sense of purpose in later life could help people resist disease and recover.
He adds: 'The biological process of ageing appears to be relatively less important than fitness, acquired disease and mental attitude.'
And with millions of older people stuck at home as a result of the pandemic, there has never been a more pressing time to be active. Failing to do so could increase the risk of frailty, and make individuals more vulnerable to illnesses, including Covid-19. Keeping moving, Sir Muir says, is key.
'Try 15 minutes walking on the spot every day,' he suggests. 'Hold on to the back of a chair if you feel a bit unsteady. I'm 75 and I've just bought a home bicycle - I'm cycling 20 minutes a day. Stairs are also very good.'
But most importantly, he adds, remember that 'you are never too old. In fact, the older you are, the more important this becomes'.