By James Gallagher
Health and science correspondent
For most people, Covid-19 is a brief and mild disease but some are left struggling with symptoms including lasting fatigue, persistent pain and breathlessness for months.
The condition known as "long Covid" is having a debilitating effect on people's lives, and stories of being left exhausted after even a short walk are now common.
So far, the focus has been on saving lives during the pandemic, but there is now a growing recognition that people are facing long-term consequences of a Covid infection.
Yet even basic questions - such as why people get long Covid or whether everyone will fully recover - are riddled with uncertainty.
What is long Covid?
There is no medical definition or list of symptoms shared by all patients - two people with long Covid can have very different experiences.
However, the most common feature is crippling fatigue.
Others symptoms include: breathlessness, a cough that won't go away, joint pain, muscle aches, hearing and eyesight problems, headaches, loss of smell and taste as well as damage to the heart, lungs, kidneys and gut.
Mental health problems have been reported including depression, anxiety and struggling to think clearly.
It can utterly destroy people's quality of life. "My fatigue was like nothing I've experienced before," said one sufferer Jade Gray-Christie,
Long Covid is not just people taking time to recover from a stay in intensive care. Even people with relatively mild infections can be left with lasting and severe health problems.
"We've got no doubt long Covid exists," Prof David Strain, from the University of Exeter, who is already seeing long-Covid patients at his Chronic Fatigue Syndrome clinic, told the BBC.
How many people are getting it?
A study of 143 people in Rome's biggest hospital, published in the Journal of the American Medical Association, followed hospital patients after they were discharged.
It showed 87% had at least one symptom nearly two months later and more than half still had fatigue.
However, such studies focus only on the minority of people who end up needing hospital treatment.
The Covid Symptom Tracker App - used by around four million people in the UK - found 12% of people still had symptoms after 30 days. Its latest, unpublished data, suggests as many as one in 50 (2%) of all people infected have long-Covid symptoms after 90 days.
Do you need severe Covid to get long Covid?
It appears not.
Half of people in a study in Dublin still had fatigue 10 weeks after being infected with coronavirus. A third were physically unable to return to work.
Crucially, doctors found no link between the severity of the infection and fatigue.
However, extreme exhaustion is only one symptom of long Covid.
Prof Chris Brightling, from the University of Leicester and the chief investigator in the PHOSP-Covid project which is tracking people's recovery, believes people who developed pneumonia may have more problems because of damage to the lungs.
How is the virus causing long Covid?
There are lots of ideas, but no definitive answers.
The virus may have been cleared from most of the body, but continues to linger in some small pockets.
"If there's long-term diarrhoea then you find the virus in the gut, if there's loss of smell it is in the nerves - so that could be what's causing the problem," says Prof Tim Spector, from King's College London.
The coronavirus can directly infect a wide variety of cells in the body and trigger an overactive immune response which also causes damage throughout the body.
One thought is the immune system does not return to normal after Covid and this causes ill-health.
The infection may also alter how people's organs function. This is most obvious with the lungs if they become scarred - long-term problems have been seen after infection with Sars or Mers, which are both types of coronavirus.
But Covid may also alter people's metabolism. There have been cases of people struggling to control their blood sugar levels after developing diabetes as a result of Covid, and Sars led to changes in the way the body processed fats for at least 12 years.
There are early signs of changes to brain structure, but these are still being investigated. And Covid-19 also does strange things to the blood, including abnormal clotting, and damaging the network of tubes that carry blood around the body.
Prof Strain told the BBC: "The theory I'm working on is a premature ageing of the small blood vessels that deliver oxygen and nutrients to the tissues." But he warned that until we figure out what is causing long Covid "it is difficult to figure out treatments."
Is this unusual?
Post-viral fatigue or a post-viral cough are well documented and common - we've probably all had an infection that has taken ages to fully recover from.
Around one in 10 people with glandular fever has fatigue which lasts for months. And there have even been suggestions that flu, particularly after the 1918 pandemic, may be linked to Parkinson's-like symptoms.
"With Covid there seem to be more far-reaching symptoms and the number of people seems to be much greater," says Prof Brightling.
The emphasis though is on the word "seems" as until we have a true picture of how many people have been infected we won't know exactly how common these symptoms are, he says.
He told the BBC: "The uniqueness of the way the virus attacks the host and the different ways it then alters the way cells behave seem to be both giving people more severe infection than other viruses and persistent symptoms."
Will people fully recover?
The number of people with long-Covid appears to be falling with time.
However, the virus emerged only at the end of 2019 before going global earlier this year so there is a lack of long-term data.
"We've asked, deliberately, to follow people for 25 years, I certainly hope only a very small number will have problems going beyond a year, but I could be wrong," said Prof Brightling.
However, there are concerns that even if people appear to recover now, they could face lifelong risks.
People who have had chronic fatigue syndrome are more likely to have it again and the concern is that future infections may cause more flare-ups.
"If long Covid follows the same pattern I'd expect some recovery, but if it takes just another coronavirus infection to react then this could be every winter," said Prof Strain.
It is still possible more problems could emerge in the future.
The World Health Organization has warned that widespread inflammation caused by coronavirus could lead to people having heart problems at a much younger age.
What should I do if I think I have long Covid?
The NHS has a "Your Covid Recovery Plan" which has advice, particularly for those who needed hospital treatment.
It recommends the "three Ps" in order to conserve energy:
- Pace yourself so you don't push yourself too hard, and make sure you have plenty of rest
- Plan your days so your most tiring activities are spread out across the week
- Prioritise - think about what you need to do and what can be put off
It advises speaking to either your hospital team or your GP if you are not recovering as quickly as you might expect.
Some have raised concerns that there is not enough support for people with long-Covid.