"Long Covid" - the long-lasting impact of coronavirus infection - may be affecting people in four different ways, according to a review.
And this could explain why some of those with continuing symptoms are not being believed or treated.
There could be a huge psychological impact on people living with long-term Covid-19, the National Institute for Health Research report says.
They need more support - and healthcare staff require better information.
Most people are told they will recover from mild coronavirus infections within two weeks and from more serious disease within three.
But the report says thousands could be living with "ongoing Covid".
And with coronavirus cases rising across the UK, this number is also likely to increase in the coming months.
Based on interviews with 14 members of a long-Covid support group on Facebook and the most recent published research, the review found recurring symptoms affecting everything from breathing, the brain, the heart and cardiovascular system to the kidneys, the gut, the liver and the skin.
These symptoms may be due to four different syndromes:
- permanent organ damage to the lungs and heart
- post-intensive-care syndrome
- post-viral fatigue syndrome
- continuing Covid-19 symptoms
Some of those affected have had a long stay in hospital with severe Covid-19 - but others, who have had a mild infection, have never even been tested or diagnosed.
The review says coming up with a "working diagnosis for ongoing Covid-19" would help people access support.
"It is becoming clear that, for some people, Covid-19 infection is a long-term illness," the report says.
"For some, this is related to their rehabilitation following a hospital admission - but others are reporting life-changing experiences that follow an initial infection that they managed at home, with symptoms becoming more severe over time."
Report author Dr Elaine Maxwell said she had assumed those who had been seriously ill with Covid-19 would be affected the most and those at low risk of death were also at low risk of living with its long-term effects.
But the review found this was not the case.
"We now know that there are people with no record of having Covid who are suffering more than someone who was ventilated for several weeks," she said.
And these debilitating effects on some people could put a "significant burden on the NHS".
'My sons have taken on the cooking and cleaning'
Jo House, a lecturer at the University of Bristol, has still not returned to work more than six months after being infected.
It started with a bad cough and difficulty breathing, but this turned into crushing fatigue and headaches before heart problems and muscle pain took over.
"The other day I got up, was really dizzy, fainted and ended up in A&E," she says.
Although her racing heart and shortness of breath have improved a bit, her ongoing symptoms are still having a huge impact on her, and her family's, life.
Her partner Ash is also experiencing symptoms that won't go away. As a result, her teenage sons have had to take on all the cooking and cleaning.
"Lots of people are classed as having mild symptoms, but it's really not mild at all. We need support," she says.
Although Jo had pneumonia, she was never tested for the virus and not admitted to hospital.
"We both made wills when we were very ill. It was scary."
The report calls for support in the community alongside the recently announced one-stop hospital clinics for long Covid.
And it says ongoing Covid is likely to have a disproportionate effect on certain groups, such as black or Asian people as well as those with existing mental health problems or learning difficulties.
"Our aim is that healthcare services and staff will use this review to better understand the experiences patients have to deal with, and provide them with the access to treatment, care and support they need," Dr Maxwell added.