As a professor of infectious diseases at Liverpool School of Tropical Medicine, Paul Garner is well qualified to talk about disease.
He's also been fighting Covid-19.
"It's the worst illness I've ever had," he told an ABC reporter. "I've had dengue. I've had malaria. I've never been as ill as this and it's been really frightening because it's so unpredictable."
He was on day 59 since being diagnosed, when this interview was recorded.
"The disease comes at you when you least expect it," he said.
"You feel quite well and then suddenly in the afternoon it slaps you round the head like a cricket bat.
"The length of the illness completely floored me. You just pray every day that it will go away."
Professor Garner is still dealing with exhaustion.
"It gives you some of the symptoms that are very similar to chronic fatigue [syndrome], but I am hesitant about calling it that," he said.
"If we call it Covid fatigue we know where we are.
"I have sympathy for people with chronic fatigue syndrome now, and I believe this disease fast-tracks you into experiencing these symptoms."
Professor Garner is just one of thousands discovering how tiring Covid-19 can be.
Scientists are still uncovering the deadly secrets of the virus, and the long-term impact for those who recover from the disease is yet to be fully understood.
In serious cases there are concerns about major organ damage.
The World Health Organisation states those with less severe cases of Covid-19 should recover within two weeks, and serious cases could take up to six weeks.
But even patients with what are considered mild bouts of the virus are experiencing fatigue and breathlessness well beyond the suggested recovery period.
At Sydney's St Vincent's Hospital, it is hoped a year-long study that is getting underway will provide some answers.
A key investigator on the St Vincent's Hospital study is infectious diseases physician, Professor Greg Dore.
"What we're interested in is looking at any effects of Covid-19," he said.
"It has a broad spectrum of acute illness, from relatively mild upper respiratory type infection symptoms to very severe pneumonia and other organ effects.
"We're interested in even the milder cases, whether there is an ongoing effect on people's health, what we call a post viral fatigue; effects on people's exercise tolerance, on neuro-cognitive function, so ability to concentrate.
"We know that you can get post viral fatigue with other viruses and glandular fever is probably the classic one, but Ross River fever and other viral illnesses are well known to cause viral fatigue."
One of the patients taking part in the study is Alex Lewis, who tested positive for Covid-19. Two months later, she is still struggling.
"I was quite fit before but now I don't feel the same way," she said
"My breathing has continued to get worse and exercise is quite difficult. And it takes me, I reckon, double the amount of time to recover.
"My fatigue, it comes and goes a bit."
She is keen to know how long it will take to shake the infection.
"We still don't have any definitive answer … I'm quite interested to find out from the study if there is anything more we can know about that," Lewis said.
'A very long recovery'
US writer Fiona Lowenstein was diagnosed with Covid-19 in mid-March, and is bewildered and anxious about how long her symptoms have lasted.
"It's been a very long recovery process and very different from what I expected," she said.
She couldn't find any information about what to expect, so she started a support group, which now has thousands of members around the world.
Many have had the same experience as her, feeling weary and unwell long after they thought they'd be better.
"I thought I had fully recovered a couple of weeks ago, then I relapsed into some old symptoms, chills and sweats. And this intense feeling of fatigue," she said.
"It almost feels like I've been hit by a truck at 4pm each day.
"It became clear to me that there needed to be a base or community for people suffering from this virus where we could talk through everything.
"My inbox was flooded."
Lowenstein has now been contacted by thousands of Covid-19 patients from around the world, experiencing similar symptoms to her.