"Long Covid" is a term which, unfortunately, is going to become more familiar.
It has no clear medical definition but Professor Gail Matthews from the University of New South Wales said: "Generally, long Covid refers to people who don't recover from the acute COVID-19 infection and go on to have longer-term symptoms."
The "acute phase" of Covid can last for up to two weeks, but the illness can then linger and become long Covid. "Most people are tending to use a period for long Covid at around the two to three months mark," Prof Matthews said.
Scientists are still learning about Covid in all its aspects. The best information so far is that about 80 per cent of patients recover within a month but there are still 5 per cent of long Covid sufferers who haven't recovered after three months.
The most common symptoms are tiredness, shortness of breath, a racing heart, chest pains and "brain fog" (a feeling that your brain is a bit slow and fuzzy).
"There are probably several different syndromes that cause long Covid," Prof Matthews said.
"In other words, long Covid isn't just one thing. There can be many underlying causes for somebody still being persistently symptomatic three months-plus after infection."
Many of the people who get long Covid are patients with a bad bout of the illness in the first place, but the mystery for doctors is why some people who get very mild forms fail to recover quickly.
"The question is, 'Why aren't these people better? Why wouldn't they have recovered as they should have if they had the flu or another viral illness'?" Prof Matthews said.
The work she and her colleagues are doing at the University of New South Wales is meant to get to the bottom of that conundrum. Nearly one-third of the Covid sufferers in the study there hadn't shaken off the symptoms after 12 weeks.
"What we're concerned about is, by this stage, more people should have recovered by now because it's now after six months post-infection," she said.
We really don't understand why fit, young adults are still unwell. In some cases, they have evidence of myocarditis, which is inflammation of the heart. In other cases, we can't find evidence of specific organ damage, but the patient continues to experience intense fatigue.Professor Gail Matthews
It's not just elderly patients who fail to make a swift recovery. The UNSW study is of long Covid patients aged from 18 upwards, with the average in the mid-40s.
"We really don't understand why fit, young adults are still unwell. In some cases, they have evidence of myocarditis, which is inflammation of the heart. In other cases, we can't find evidence of specific organ damage, but the patient continues to experience intense fatigue, almost like a chronic fatigue type syndrome," Prof Matthews said.
Do all people eventually recover?
It's too soon to know how long long Covid may be - or even whether it may be everlasting Covid.
Professor Matthews said most studies showed perhaps 20 per cent of sufferers may not get back to full health - but Covid hasn't been around long enough to know how it will play out.
A study in Britain "suggests that among those who needed hospital treatment for the initial illness, it is common for it to last five months or more, and there are separate reports of it lasting 12 months or more," according to the British Heart Foundation.
Who is most likely to get it?
"Older people, women, and those who had five or more symptoms in the first week of becoming ill with COVID-19 were more likely to develop long Covid," according to a study cited by the British Heart Foundation.
"They found that long Covid affects around 10 per cent of 18- to 49-year-olds who get COVID-19, increasing to 22 per cent of people over 70."
The researchers also found people with asthma were also more likely to develop long Covid.
Another study also found long Covid tended to increase with age and was more likely to impact women. It found long Covid was higher among people who were overweight or obese, who smoked, lived in deprived areas or had Covid so badly they had to be admitted to hospital.
"We've seen from the early data coming out that men were at much more risk of very severe disease and sadly of dying from Covid. It appears that women are more at risk of long Covid," Dr Claire Steves, from Kings College London, told the BBC.
It seems children can get long Covid but the numbers are not known yet.
Doctors in Rome interviewed 129 Covid sufferers aged from six to 16 who had been diagnosed with COVID-19. They reported more than one-third had one or two lingering symptoms at least four months after infection. A further quarter had three or more symptoms like insomnia and fatigue (similar to those in adults).
How do you know if you've got it?
By the symptoms, of course - but the symptoms are common to other illnesses. It might be something else you've caught.
There is no test - instead it is "diagnosis of exclusion". Doctors rule out other possibilities, like diabetes or iron deficiency.
There are no proven drug treatments so the main effort is to manage symptoms so people get back to health gradually.
It seems vaccination does help. About half the people with long Covid reported an improvement in their symptoms after getting a jab.
But more research is needed. Vaccination does help prevent people from getting the illness in the first place.