In early 2020, when Covid-19 was spreading rapidly across the globe, the data suggested that it was a short-lived infection.
In February that year, the World Health Organisation (WHO) reported that mild cases lasted two weeks on average, and more severe cases three to six. But some patients soon attested to feeling unwell for far longer.
The term "Long Covid" seems to have been first used in May 2020 by an academic from hard-hit Lombardy in Italy, discussing her symptoms on Twitter. It "has a strong claim to be the first illness created through patients finding one another on Twitter", reported the journal Social Science & Medicine. "It moved from patients, through various media, to formal clinical and policy channels in just a few months."
By August of 2020, the WHO was dealing with Long Covid advocacy groups.
There are no tests or biological markers for Long Covid, and there's no universally agreed definition (even the name is contested: some use "long-haul Covid" or "post-Covid-19 syndrome").
However, the WHO, the UK's National Institute for Health and Care Excellence, and other such health bodies, diagnose it in a similar way: with reference to new symptoms which persist two to three months after an initial Covid infection, and which can't be explained by another diagnosis.
The most commonly reported symptoms are fatigue and memory problems, or "brain fog", but there are many others, ranging from mood disorders to shortness of breath, loss of smell and/or taste, kidney damage, joint aches and cardiovascular issues.
According to an overview published in Nature in January, at least 65 million people around the world, at a conservative estimate, currently have Long Covid in one form or another, and the number is growing all the time. It occurs in an estimated 10-30% of non-hospitalised Covid cases, and 50-70% of hospitalised cases.
Vaccination may provide partial protection: it occurs in an estimated 10-12% of cases in people who have been vaccinated.
The highest percentage of diagnoses have occurred among people aged between 36 and 50 who have had an acute case of Covid without having to go to hospital. But women, older people, and people who've been hospitalised for Covid are significantly more likely to have long-lasting symptoms.
Doctors don't know. It's not a single disease; the Covid virus attacks the body in many different ways, with many different effects - particularly in severe cases - and it also interacts with pre-existing conditions. But researchers are investigating a number of hypotheses.
Among the theories being looked at are persistent reservoirs of the Covid virus in patients' bodies; microscopic clotting affecting blood vessels; a misfiring immune system attacking healthy cells; misfiring signals in the brain stem or vagus (cranial) nerve; and problems relating to earlier viral infections that can reactivate when the immune system is under strain.
In particular, it seems that it may reactivate common viruses such as Epstein-Barr (the cause of glandular fever) and HHV-6 (the cause of roseola). This suggests some overlap between Long Covid and other, similarly poorly understood post-viral syndromes.
The best-known is myalgic encephalomyelitis (ME), also known as chronic fatigue syndrome (CFS). According to one study, half of those affected by Long Covid also meet the diagnostic criteria for ME/CFS.
There are similarities, too, to other post-infection syndromes, such as those linked to the ebola and chikungunya viruses, and to postural orthostatic tachycardia syndrome (POTS), a heart problem sometimes brought on by Covid and other viral infections.
Because the mechanisms behind ME/CFS and POTS are still unclear, they have sometimes been treated as psychosomatic complaints, leading to conflict between the medical profession and activist patient groups.
Many health systems, including the NHS, offer specialised clinics and advice, but there is currently no effective, one-size-fits-all treatment.
A typical Long Covid clinic is a multidisciplinary enterprise involving physiotherapists, occupational therapists and psychologists as well as medical specialists. "Pacing", a form of "energy management" (balancing activity and rest) developed for ME/CFS, can help people with chronic fatigue to resume normal activities.
Clinical trials are under way for symptom-specific drug treatments, such as anticoagulants for abnormal clotting and naltrexone for neuroinflammation, as well as various antiviral drugs. Recovery times vary greatly. But for a majority of patients - 75%, according to a recent study in the European Respiratory Journal - Long Covid clears up within 12 months.
The Office for National Statistics estimates that two million people in the UK, or 3% of the population, had self-reported Long Covid symptoms in January this year. People living in deprived areas are among the most likely to be affected.
A recent BBC investigation found that up to 10,000 NHS staff were off sick with Long Covid. Last summer, the Institute for Fiscal Studies calculated that 110,000 people were missing from work as a result of Long Covid, costing the economy £1.5bn per year in lost earnings.
As long as Covid is still out there - the latest UK figures, for May, show about 60,000 weekly confirmed cases - Long Covid will be a threat. On the bright side, though, the ONS figures suggest that the risk of developing the condition after a second or third infection is significantly lower than after the first.