(CNN) In the hours after President Trump confirmed he had Covid-19, his symptoms were described as mild.
Even once he had a fever and a cough, they were still mild.
But one of the factors that makes Covid-19 complicated is how quickly it can change.
Trump's case progressed rapidly from mild to severe, according to doctors who treat Covid-19 patients. They based that assessment on details shared by the President's medical team; Trump was hospitalized and treated with an experimental monoclonal antibody treatment, the antiviral remdesivir and the corticosteroid dexamethasone, which is recommended for use in Covid-19 patients who are receiving supplemental oxygen.
All coronavirus infections start mild, doctors say, and the majority -- about 80% -- stay that way.
Mild infection, though, is a broad description that captures a number of possible symptoms. And with Covid-19, even a mild case can be lengthy and uncomfortable.
Mild symptoms of Covid-19 can include fever, cough, sore throat, malaise, headache, muscle pain, congestion or runny nose, loss of taste or smell, and diarrhea, according to the US Centers for Disease Control and Prevention.
"Things that you would not feel well with, but not necessarily present to the hospital with," Dr. Rochelle Walensky, the chief of the Division of Infectious Diseases at Massachusetts General Hospital, told CNN.
"You generally don't have shortness of breath, you don't have an abnormal chest X-rays or just imaging," Walensky said. " ... So, it's people who said, 'You know, I bet you're unwell, why don't you stay home. Sounds like you can be safely cared for at home.' That's mild, mild illness."
There are asymptomatic cases of Covid-19, where people have no symptoms, or symptoms so mild they're unrecognizable. But people who are asymptomatic when they test positive may still develop symptoms in the days ahead -- and they can still spread the virus.
"If you're diagnosed with Covid, you're told go home, stay there and don't bother coming to the hospital until you start having shortness of breath," Dr. Carlos del Rio, an infectious disease expert at Emory School of Medicine, told CNN.
"They don't have shortness of breath," del Rio said of people with mild illness -- but they should monitor their oxygen levels for any indication that they could be progressing to a more severe form of the disease.
"The great majority of people with mild (symptoms), stay as mild," del Rio added.
Mild may not be easy, though. Some patients experience Covid-19 symptoms long after they would expect to feel better, and even after they've stop testing positive for it.
The infection can turn into a prolonged illness, even among young adults without underlying health conditions. The CDC reported in July that 35% of those surveyed by the agency said they still weren't back to normal more than three weeks after first testing positive for the virus.
"You can have mild Covid and still be a long hauler," del Rio said, referring to people who experience mild coronavirus symptoms -- such as fever, cough, sore throat, muscle pain and loss of taste or smell -- for weeks or months after testing positive for the virus.
As Covid-19 progresses, a patient might start to show evidence of lower respiratory disease, but they still have good oxygenation, infectious disease specialist Walensky said. That's moderate illness.
"You're starting to have shortness of breath, starting to have abnormal chest X-rays but your oxygenation level is over 94% and generally that means you don't need supplemental oxygen. That's moderate," she said.
The National Institutes of Health recommends closely watching patients with moderate forms of the disease.
"Given that pulmonary disease can rapidly progress in patients with COVID-19, close monitoring of patients with moderate disease is recommended," the agency said on its website.
Not all moderate cases of Covid-19 look the same.
Some people will not need to be hospitalized for treatment, del Rio said. But others will. And from there, the illness can become severe.
The risk factors for severe Covid-19 include older age, lung disease or heart disease, or obesity, according to the CDC.
"The older you get, the more likely you are to have severe disease," del Rio noted.
"Severe is when you start getting short of breath and you can really tell that you're short of breath because you're breathing quickly, generally over 30 times a minute, and your oxygenation is less than 94%, and that generally means you need supplemental oxygen," Walensky said.
The NIH said severely ill Covid-19 patients "may experience rapid clinical deterioration."
"Once the President's oxygen saturation went below 94%, he went from having mild disease to having severe disease," del Rio explained.
But, del Rio noted, that hasn't been the case for every case linked to the White House -- first lady Melania Trump, who described her symptoms as "mild" early on, has not been hospitalized.
"Both started with mild illness. She has remained with mild illness, as far as we can tell, but he went on to develop severe disease," del Rio said, noting that the first lady is 50, while the President is 74.
"Thirty percent of people who are in the President's demographic, 74 years old, need to be hospitalized and about 10% of those need to be in an ICU," Walensky said.
The coronavirus can also progress to what the NIH categorizes as critical illness.
"Critical Illness would be those who have signs of respiratory failure and need a breathing machine," Walensky said. Patients may experience other organ dysfunction and shock, she said.
"That's critical, generally ICU-level care."
"Severe cases of COVID-19 may be associated with acute respiratory distress syndrome ... cardiac dysfunction, elevations in multiple inflammatory cytokines that provoke a cytokine storm, and/or exacerbation of underlying comorbidities. In addition to pulmonary disease, patients with COVID-19 may also experience cardiac, hepatic, renal, and central nervous system disease," the NIH said in its description of critical Covid-19 disease.
Critically ill patients with Covid-19 all started with a mild form of the disease and progressed to needing ICU care, Walensky said.
And for those critically ill patients who eventually recover, they progress in reverse -- shifting from critical to severe, then into moderate illness and finally back to a mild form of the sickness, she said.