The first cases of long Covid emerged in late spring last year, just as lockdowns were beginning to bring the initial wave of the virus under control.
People who should have fully recovered were still suffering weeks later.
Some continued to have shortness of breath, or issues such as a loss of taste or smell, palpitations, chest pain, brain fog, or a host of other issues.
12 months on, exhaustion is the most common symptom, sometimes accompanied with tingling or numbness in hands and feet.
Most had only had a mild case of the virus, and the vast majority of sufferers were women.
They also tended to be younger, between the ages of 20 and 50.
As new cases mounted, others continued to stall in their recovery.
Now, more than one year since the pandemic was declared, millions of people are believed to still be suffering from the condition.
Studies from the UK have shown that up to a quarter of people there experience long Covid.
But doctors say the condition could affect considerably more.
“We don’t really know right now how many patients will develop these long Covid symptoms after they have had this infection,” said Kristin Englund, a doctor at Cleveland Clinic, in an online explainer on the topic.
“Studies look at anything between 10 per cent and 80 per cent," she said.
“So there could be a large number of people experiencing symptoms well after that four week time period when we would expect people to normally recover.”
Zijian Chen, an endocrinologist at Mount Sinai, who was appointed as medical director at the hospital’s Center for Post-Covid Care, was shocked when he realised the full scope of patients in its database who suffered lingering effects of the virus during the first wave.
"I looked at the number of patients that were in the database and it was, I think, 1,800 patients," he told The Atlantic in a recent interview.
“I freaked out a little bit. Oh my God, there’s so many patients telling us that they still have symptoms.
“We didn’t expect this from a virus.
“We expect that with viral infections as a whole, with few exceptions, you get better.”
Those with evident issues were sorted into groups to see specialists.
Some had fibrosis or lung scarring, others developed heart problems.
But in 90 per cent of people there was no obvious cause of their ongoing health issues.
"We couldn’t see what was wrong," Dr Chen said.
The condition largely remains a mystery, but studies are beginning to reveal answers.
Long Covid recently received an official name - Post Acute Sequelae of SARS-CoV2, or PASC.
The condition is the subject of studies across the world, where clinics have been established to treat sufferers.
At least 10 long Covid clinics have been established in London alone, where doctors are struggling to see everyone who needs help.
“The referrals at the clinics are twice the number we can cope with.” Melissa Heightman, a respiratory physician at University College London Hospital’s clinic for post-Covid treatment told Bloomberg.
Dubai resident Eman Jamal has seen her fair share of doctors since she caught the virus last April.
She was pretty sick at the time, with a racking cough and pounding headaches, but was one of the lucky ones, or so she thought after she was discharged following an 18-day stay at Mediclinic Parkview Hospital in Al Barsha.
Almost a year on she still suffers from chest tightness, pain, muscle spasms and fatigue.
Ms Jamal has seen countless doctors, and although she has been diagnosed with asthma, it cannot explain the extent of her systemic health problems.
After a “terrible January”, she had herself retested for the virus - it was negative.
By that point she had no antibodies left, so reinfection was a real possibility given the prevalence of the virus at the time.
“The first time I tested for antibodies was in September of 2020. They were on the high side. I got tested again at the end of December and they were completely gone,” said the Palestinian American, 36.
“It’s different numbers in different countries, but if you have 1.4 and above you are considered to have antibodies. I had 7.6. I was really comfortable with that number. I felt I could do things and I could go out and be a little bit more social. I was still being super careful. By December I had nothing.”
Vaccine's positive impact
Ms Jamal rode out the start of the year and in February things returned to her “new normal” of bad days and good days. Then towards the end of the month she got her Covid-19 vaccine.
She was diagnosed with asthma after suffering from the virus, so qualified for Pfizer under Dubai’s priority system.
“I was in bed for 72 hours. I had a fever of 39.5C. I was weak. I was shivering,” she said.
But after a rough few days, she started to feel better. Her long Covid symptoms began to subside.
“There was a noticeable improvement about 11 or 12 days after I took it,” she said.
Other sufferers have also reported a vast improvement in their symptoms after they got vaccinated.
Scientists do not fully understand it yet, but they have some theories.
One explanation could be that Covid long haulers are still harbouring the virus somewhere in their bodies, where it is sheltered from the immune system, like the brain, and it is replicating slowly.
Studies have shown the virus can indeed infect the brain, where it is believed to enter via the nose.
This is what an autopsy study concluded last November, when researchers discovered coronavirus particles intact in cells located at the roof of the nose, along with evidence of "active replication in the tissue".
They said from there the virus could access the olfactory bulb – the neural structure in the front of the brain involved in the sense of smell – and travel via specific cranial nerves deep into the brain where it still replicates.
If symptoms are indeed caused by a “viral ghost,” antibodies produced in response to a vaccine may be able to "eliminate the reservoir", Akiko Iwasaki, an immunologist at Yale University, wrote on Twitter.
Alternatively, long Covid symptoms could also be caused due to an autoimmune response to the initial infection, she said.
If symptoms are due to an autoimmune reaction, the vaccine may “divert the autoimmune cells,” said Ms Iwasaki.
Ms Jamal does not know whether the improvement is related to the vaccine, or even if it will last, but she is hopeful.
“I can’t confidently say it was the vaccine only. But something is getting better.
“My next dose is this coming Sunday,” she said.
“I am hoping the improvement continues.”