What began as a familiar flu-like cluster of chills, headaches and fever has rapidly expanded over the last three months into a catalogue of syndromes affecting almost all the body's organs, from the brain to the kidneys.
The new coronavirus can also push the immune system into overdrive, unleashing an indiscriminate assault -- known as a cytokine storm -- on pathogens and their human hosts alike.
"Most viruses can cause disease in two ways," explained Jeremy Rossman, a senior lecturer in virology at the University of Kent.
"They can damage tissue where the virus replicates, or they can cause damage as a side-effect of the immune system fighting off the disease."
Doctors suspect, for example, that COVID-19 is behind the hospitalisation in recent weeks of several dozen children in New York, London and Paris diagnosed with a rare inflammatory disorder similar to toxic shock syndrome.
Affecting mainly young children, the painful disease attacks artery walls and can cause organ failure.
Dozens of medical studies in recent weeks have detailed other potentially lethal impacts including strokes and heart damage.
Researchers from the urology department of Nanjing Medical University, writing this week in Nature Reviews, described patients developing severe urinary complications and acute kidney injury.
They also observed "dramatic changes" in male sex hormones.
- '1-in-10,000 still a lot' -
"After recovery from COVID-19, young men who are interested in having children should receive a consultation regarding their fertility," they concluded.
Does that mean that COVID-19 causes a uniquely broad array of symptoms? Not necessarily, virologists and other experts say.
"If it is a common disease, then even rare complications will happen frequently," Babak Javid, a consultant in infectious diseases at Cambridge University Hospitals, told AFP.
There are nearly 3.8 million confirmed COVID-19 cases around the world, but the true number of infections -- taking into account undetected and asymptomatic infection -- "is going to be in the tens, possibly hundreds of millions," he said.
"So if one-in-1,000, or even one-in-10,000, get complications, that is still thousands of people."
Some of the rarer symptoms associated with COVID-19 are also known to have been triggered by influenza, which kills several hundred thousand people worldwide every year, he noted.
For the new coronavirus, frontline general practitioners across the globe have been the first to look for patterns in the unfolding pandemic.
"At the outset, we were told to watch out for headaches, fever and a light cough," recalls Sylvie Monnoye, a family doctor in central Paris for nearly three decades.
"Then they added a runny nose and a scratchy throat. After that, digestive problems, including stomach aches and severe diarrhoea."
The list kept growing: skin lesions, neurological problems, sharp chest pains, loss of taste and smell.
- A feeling of confusion -
"We started to think that we should suspect everything," Monnoye said, dressed from head-to-toe in protective wear.
Some patients were so terrified, she added, that they cowered in the corner of her office afraid to touch anything or get too close to her.
An internal US Centers for Disease Control (CDC) report with a breakdown of symptoms for 2,591 COVID-10 patients admitted to hospital between March 1 and May 1 chimes with such anecdotal accounts.
Three-quarters of the patients experienced chills, fever and/or coughing, with nearly as many showing shortness of breath.
These are, by far, the most common COVID-19 symptoms.
Nearly a third complained of flu-like muscle aches, while 28 percent experienced diarrhoea and a quarter nausea or vomiting, according to the internal report, leaked to the media.
Some 18 percent had headaches, while 10 to 15 percent were hit by chest or abdominal pain, runny nose, sore throat and/or a feeling of confusion.
Less than one percent of the CDC cohort had other symptoms, including seizures, rashes and conjunctivitis.
Health authorities have been slow in alerting the public to this panoply of possible impacts.
- Loss of smell -
Until the end of April, the CDC itself only listed three on it's website: coughing, fever and shortness of breath. The update included only a few more: chills, muscle pain, headaches and loss of smell or taste. France's health officials made a similar update on May 5.
A loss of smell and taste was found in only 3.5 percent of patients included in the CDC report, but experts suspect these symptoms are -- for reasons unknown -- far more prevalent in less severe cases where people were not hospitalised.
Monnoye said it was among the most common of the symptoms she encountered, and agreed that it was "probably linked to a milder form of the disease".
"I don't have any patients with these symptoms who had serious complications," she said.
The loss of taste and smell, experts note, is extremely rare with other types of virus.
Another cluster of symptoms rarely found with in flu patients appears to arise from blood clots.
Heart problems, liver thrombosis, lung embolisms and brain damage in COVID-19 patients have been traced to such clots in a flurry of recent studies. Others have described kidney failure and even gummed-up dialysis machines.
"When one is very sick with COVID, you can have a problem with blood clots forming, and that seems to be much, much more common than with other viral infections," added Javid.
"Compared to influenza, you are much more likely to become seriously ill, and to die."
