As more people recover from COVID-19, many will find their ordeal may not end when the infection is over.

With the disease emerging in China only several months ago, doctors continue to try to get a better understanding of how the new coronavirus impacts long-term health.

It's definitely different from the flu.

More than a third of of patients, or 35%, not sick enough to be hospitalized still weren't back to normal up to three weeks after testing positive, according to a survey published by the Centers for Disease Control and Prevention in July. Those who had lingering symptoms mostly complained about fatigue, cough and headache.

In contrast, more than 90% of flu patients recover within two weeks, the report noted.

"COVID-19 can result in prolonged illness, even among young adults without underlying chronic medical conditions," the authors wrote.

Some patients have been called "long haulers" as they continue to have symptoms for months after being infected with the coronavirus, perhaps because of the inflammation triggered by COVID-19.

Doctors aren't sure when those problems will subside.

"We really don't know," Dr. Robert Glatter, an emergency medicine physician at Lenox Hill Hospital in New York, told TODAY.

"There are many patients that have gone on for months now, since mid-March, that I've treated in the emergency department that continue to have fever, aches, chills, they're short of breath, they're having chest pain, they're having generalized fatigue."

It's also clear that survivors of the severe type of the illness face a complicated health picture, and not just when it comes to their lungs. COVID-19 seems to be more than a respiratory disorder, with people also experiencing a gastrointestinal version of the disease.

Doctors are trying to figure out any long-term impact on the heart and other organs, said Dr. Andrew Freeman, a cardiologist at National Jewish Health in Denver, Colorado, and a member of the American College of Cardiology’s COVID-19 Response Work Group.

“We’re going to see enormous populations of people who have convalesced — who have survived the virus. Then the question is: Do we need to give them echocardiograms? Do we need to do other things to surveil for whatever the long-term (consequences) are?” Freeman said.

Here’s what doctors have seen so far:

Lungs

COVID-19 patients who developed acute respiratory distress syndrome — a life-threatening lung injury due to infection — and had to be hospitalized in the intensive care unit are more likely to have long-term consequences, said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins University Center for Health Security in Baltimore and a spokesman for the Infectious Diseases Society of America.

“There are people who are going to have scarring in their lungs from what’s happened and that may not be completely reversible,” Adalja told TODAY. “It’s not just with coronavirus; we see this with all types of pneumonia that lead to ARDS.”

A 28-year-old woman recently became the first COVID-19 patient in the U.S. to receive double lung transplant. She had such severe and permanent lung damage that she would have died without the operation, her surgeon said. She was discharged from Northwestern Memorial Hospital in Chicago in July.

Even patients who are less sick may have diminished lung function that’s going to persist, including decreased exercise capacity that leaves them short of breath.

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Some COVID-19 survivors had a 20-30% drop in lung function after recovery and “gasp if they walk a bit more quickly,” doctors in Hong Kong told the South China Morning Post.

In such cases, cardiopulmonary rehab may help in rebuilding strength and capacity, though a person may not return completely all the way back to baseline, Adalja said.

Heart

About 20% of patients with COVID-19 in China had heart damage during hospitalization, a study found.

Another study discovered about 16% patients developed arrhythmia, while other reports indicated cases of acute onset heart failure, heart attack and cardiac arrest after coronavirus infection, the American College of Cardiology warned.

People with the severe form of the illness can also develop myocarditis, an inflammation of the heart muscle, and sometimes don’t fully recover by the time they’re discharged from the hospital, Freeman noted.

Two studies published in July showed the virus can linger in the heart for months, with many patients showing signs of ongoing inflammation, even without symptoms.

Whenever there’s enormous demand placed on the heart — in cases when someone is severely sick and on life support, for example, or dealing with an intense inflammatory response — there can always be some element of cardiac injury, Freeman explained.

Plus, any long-term lung disease can have effects on the heart, particularly its right side.

“The lungs and heart are coupled tightly,” Freeman explained. “Sometimes when the lungs are ill, the pulmonary arteries — which are what leaves the right side of the heart — can also develop inflammation, disease or a thickening.”

In addition, viral illnesses can destabilize plaque in the arteries, potentially resulting in a blockage and putting patients at risk of a heart attack, the American Heart Association warned.

Doctors have also noticed an unusual amount of blood clotting in COVID-19 patients, which raises the risk of deep vein thrombosis, strokes and heart attacks.

Kidneys

There’s no evidence COVID-19 hurts the kidneys of people who have mild to moderate infection, but kidney abnormalities have been seen in 25-50% of patients who develop the severe type of the disease, according to the International Society of Nephrology.

They have more protein and red blood cells in their urine, and about 15% of them also develop a decline in filtration function.

When patients with severe COVID-19 go into intensive care, up to half have kidney failure that requires some form of dialysis, Dr. Alan Kliger, co-chair of the American Society of Nephrology Covid-19 Response Team, told CNBC.

There are several theories why: It may be that the coronavirus directly attacks the kidneys, or that the blood clots damage them, or that the organs don't get enough oxygen during the illness.

The new coronavirus “is an infectious organism and can lead to a cascade of immune changes that lead to sepsis, and sepsis is characterized by multiple organ systems being compromised,” Adalja said. “Some individuals with sepsis can get acute kidney injury.”

The long-term health effect of this on COVID-19 survivors is not known, the International Society of Nephrology noted.

Brain and mental health

The longer patients have to remain in the ICU, the more likely they are to suffer long-term cognitive and emotional effects of being sedated. Doctors call it "post-intensive care syndrome" or post-ICU delirium, and describe it as a type of post-traumatic stress.

"Often when patients come out of the ICU, they really struggle to think as clearly as they did before," Dr. Amy Bellinghausen, a pulmonary, critical care and sleep medicine fellow at the University of California, San Diego, told NBC News.

She estimated up to two-thirds of ventilated patients may be affected. Possible causes include not getting enough oxygen or blood to the brain, or the medications used to sedate a patient.

But even when people aren't on a ventilator, scientists are worried about COVID-19-related brain damage that may lead to psychosis, delirium and cognitive deficits. NBC News correspondent Morgan Chesky was frustrated by "mental fog" after his diagnosis, noting he found it hard to focus on and process even basic tasks.

The stress of the disease can trigger hair loss as part of a temporary condition known as telogen effluvium.

Nervous system

Neurological symptoms are showing up, too. Other coronaviruses that affect humans can invade the central nervous system, so it makes sense COVID-19 may have neurological manifestations, Dr. Kenneth Tyler, chair of neurology at University of Colorado School of Medicine in Aurora, told Neurology Today.

Indeed, a study found neurologic symptoms were seen in 36% of 214 COVID-19 patients in China, including dizziness, headache and taste and smell impairment. Some can last for weeks.

Another study, published in the journal Brain in July, found a high incidence of acute disseminated encephalomyelitis among COVID-19 patients, which the authors called "striking." The condition damages white brain matter, which may lead to vision loss, weakness to the point of paralysis and difficulty coordinating muscle movements, according to the National Institute of Neurological Disorders and Stroke.

Neurological side effects have been seen in children, too.

Bottom line:

Doctors are still trying to understand any effects that are unique to the new coronavirus.

"Long haulers," the COVID-19 survivors with lingering problems, should especially pay attention to breathing problems, chest pain and fatigue. "It's very important that patients continue to receive care from a pulmonologist or primary care doctor," said Dr. Roger Alvarez, a pulmonologist at the University of Miami Health System, in a statement.

Doctors also urged families to watch for unusual cognitive changes in their loved ones, including lapses in consciousness and unexplained confusion. Knowing symptoms of a stroke or heart attack are always important and especially so during the pandemic.

“There may be some differences in the way the immune system reacts to this (virus),” Adalja said. “We’ll only learn that from long-term studies of survivors.”