After any coronavirus disease 2019 (COVID-19) illness, no matter how serious, some people report that symptoms stay for months. This lingering illness has often been called long COVID or post-COVID-19 syndrome. You might hear it called long-haul COVID or post-acute sequelae of SARS-CoV-2 (PASC).

There is no universal definition of long COVID right now.

In the U.S., some experts have defined long COVID as a long-lasting, called chronic, condition triggered by the virus that causes COVID-19. The medical term for this is an infection-associated chronic condition.

As researchers learn more about long COVID, this definition may change.

It can be hard to get care for long COVID. Treatment may be delayed while you work with healthcare professionals. And people with long COVID may have their health problems dismissed by others, including healthcare professionals, family members or employers.

For people with cultural or language barriers, getting a long COVID diagnosis can be even harder. Pulling together information about symptoms and timing can be a challenge too. This is especially true when medical history is fragmented or when someone is managing symptoms related to memory or that affect the thought process.

Underdiagnosis may be more common among people who have less access to healthcare or who have limited financial resources.

If you're having long COVID symptoms, talk with your healthcare professional. It can help to have your medical records available before the appointment if you are starting to get care at a new medical office.

To prepare for your appointment, write down:

• When your symptoms started and if they come and go.
• What makes your symptoms worse.
• How your symptoms affect your activities.
• Questions you have about your illness.

List medicines and anything else you take, including nutrition supplements and pain medicine that you can get without a prescription. Some people find it helpful to bring a trusted person to the appointment to take notes.

Keep visit summaries and your notes in one place. That can help you track what actions you need to take or what you've already tried to treat your symptoms.

Also, you might find it helpful to connect with others in a support group and share resources.

The conditions linked as part of long COVID may get better over months or may last for years.

Current research has found that long COVID is a chronic condition triggered by the virus that causes COVID-19. The medical term for this is an infection-associated chronic condition.

Researchers don't know exactly how COVID-19 causes long-term illness, but they have some ideas. Theories include:

• The virus that causes COVID-19 upsets immune system communication. This could lead immune cells to mistake the body's own cells as a threat and react to them, called an autoimmune reaction.
• Having COVID-19 awakens viruses that haven't been cleared out of the body.
• The coronavirus infection upsets the gut's ecosystem.
• The virus may be able to survive in the gut and spread from there.
• The virus affects the cells that line blood vessels.
• The virus damages communication in the brain stem or a nerve that controls automatic functions in the body, called the vagus nerve.

Because the virus that causes COVID-19 continues to change, researchers can't say how many people have been affected by long COVID. Some researchers have estimated that 10% to 35% of people who have had COVID-19 went on to have long COVID.

Long COVID makes life more difficult for many people. To provide better options for care, research is going on to better understand this illness. In the meantime, adults or children with long COVID may be able to get support for daily activities affected by the illness.

In research studies, more than 200 symptoms have been linked to long COVID. Symptoms may stay the same over time, get worse, or go away and come back.

Common symptoms of long COVID include:

• Extreme tiredness, especially after activity.
• Problems with memory, often called brain fog.
• A feeling of being lightheaded or dizzy.
• Problems with taste or smell.

Other symptoms of long COVID include:

• Sleep problems.
• Shortness of breath.
• Cough.
• Headache.
• Fast or irregular heartbeat.
• Digestion problems, such as loose stools, constipation or bloating.

Some people with long COVID may have other illnesses. Diseases caused or made worse by long COVID include migraine, lung disease, autoimmune disease and chronic kidney disease.

Diseases that people may be diagnosed with due to long COVID include:

• Heart disease.
• Mood disorders.
• Anxiety.
• Stroke or blood clots.
• Postural orthostatic tachycardia syndrome, also called POTS.
• Myalgic encephalomyelitis-chronic fatigue syndrome, also called ME-CFS.
• Mast cell activation syndrome.
• Fibromyalgia.
• Diabetes.
• Hyperlipidemia.

People can get long COVID symptoms after catching the COVID-19 virus even if they never had COVID-19 symptoms. Also, long COVID symptoms can show up weeks or months after a person seems to have recovered.

And while the COVID-19 virus spreads from person to person, long COVID is not contagious and doesn't spread between people.

Risk factors for long COVID are just starting to be known. In general, most research finds that long COVID is diagnosed more often in females of any age than in males. The long COVID risk also may be higher for people who have cardiovascular disease before getting sick.

Some research also shows that getting a COVID-19 vaccine may help prevent long COVID.

Many other factors may raise or lower your risk of long COVID, but research is still ongoing.

See a healthcare professional if you have long COVID symptoms. Part of long COVID's definition is symptoms that last for three months.

But at this time, no test can say whether you have long COVID. Since you may not have symptoms if you have an infection with the COVID-19 virus, you may not know you had it. Some people have mild symptoms and don't take a COVID-19 test. Others may have had COVID-19 before testing was common.

Long COVID symptoms may come and go or be constant. They also can start any time after you had COVID-19. But symptoms still need to be documented for at least three months in order for a health care professional to diagnose long COVID.

Healthcare professionals may treat your symptoms or conditions before a long COVID-19 diagnosis. And they may work to rule out other conditions over the time you start having symptoms.

Your healthcare team might do lab tests, such as a complete blood count or liver function test. You might have other tests or procedures, such as chest X-rays, based on your symptoms.

The information you give and any test results can help your healthcare professional come up with a treatment plan.

Healthcare professionals treat long COVID based on the symptoms. For tiredness, your healthcare professional may suggest that you be active only as long as your symptoms stay stable. If you start to feel worse, rest and don't push through your tiredness.

For symptoms of pain, breathlessness or brain fog, work with your healthcare professional to find a treatment plan that works for you. That may include medicine you can get without a prescription for pain, prescription medicine, supplements and referrals to other healthcare team members.

For loss of taste or smell, a process to retrain the nerves involved in those processes may help some people. The process is called olfactory training. For people with POTS or a fast heartbeat, the healthcare professional may suggest prescription medicine as well as a plan to stay hydrated.

Treatment for other long COVID symptoms may be available so contact your healthcare professional for options.

Post-COVID-19 syndrome involves a variety of new, returning or ongoing symptoms that people experience more than four weeks after getting COVID-19. In some people, post-COVID-19 syndrome lasts months or years or causes disability.

Research suggests that between one month and one year after having COVID-19, 1 in 5 people ages 18 to 64 has at least one medical condition that might be due to COVID-19. Among people age 65 and older, 1 in 4 has at least one medical condition that might be due to COVID-19.

The most commonly reported symptoms of post-COVID-19 syndrome include:

• Fatigue
• Symptoms that get worse after physical or mental effort
• Fever
• Lung (respiratory) symptoms, including difficulty breathing or shortness of breath and cough

Other possible symptoms include:

• Neurological symptoms or mental health conditions, including difficulty thinking or concentrating, headache, sleep problems, dizziness when you stand, pins-and-needles feeling, loss of smell or taste, and depression or anxiety
• Joint or muscle pain
• Heart symptoms or conditions, including chest pain and fast or pounding heartbeat
• Digestive symptoms, including diarrhea and stomach pain
• Blood clots and blood vessel (vascular) issues, including a blood clot that travels to the lungs from deep veins in the legs and blocks blood flow to the lungs (pulmonary embolism)
• Other symptoms, such as a rash and changes in the menstrual cycle

Keep in mind that it can be hard to tell if you are having symptoms due to COVID-19 or another cause, such as a preexisting medical condition.

It's also not clear if post-COVID-19 syndrome is new and unique to COVID-19. Some symptoms are similar to those caused by chronic fatigue syndrome and other chronic illnesses that develop after infections. Chronic fatigue syndrome involves extreme fatigue that worsens with physical or mental activity, but doesn't improve with rest.

You might be more likely to have post-COVID-19 syndrome if:

• You had severe illness with COVID-19, especially if you were hospitalized or needed intensive care.
• You had certain medical conditions before getting the COVID-19 virus.
• You had a condition affecting your organs and tissues (multisystem inflammatory syndrome) while sick with COVID-19 or afterward.

Post-COVID-19 syndrome also appears to be more common in adults than in children and teens. However, anyone who gets COVID-19 can have long-term effects, including people with no symptoms or mild illness with COVID-19.

If you're having symptoms of post-COVID-19 syndrome, talk to your health care provider. To prepare for your appointment, write down:

• When your symptoms started
• What makes your symptoms worse
• How often you experience symptoms
• How your symptoms affect your activities

Your health care provider might do lab tests, such as a complete blood count or liver function test. You might have other tests or procedures, such as chest X-rays, based on your symptoms. The information you provide and any test results will help your health care provider come up with a treatment plan.

In addition, you might benefit from connecting with others in a support group and sharing resources.