It has been more than one year since Rebekah Hogan got COVID-19. She still suffers from pain and tiredness. She struggles with thinking and remembering. Her condition makes her unable to do her nursing job or do normal household activities.

Hogan's husband and three children also have signs of the condition. The family lives in Latham, New York.

Her experience with COVID-19 has her questioning her worth as a wife and mother.

"Is this permanent? Is this the new norm?" said the 41-year-old Hogan. "I want my life back."

There are estimates that more than one-third of people infected with COVID-19 will develop long-lasting problems. Scientists are trying to understand the cause of so-called "long COVID" and find treatments for it.

It is too soon to know whether people infected with the fast-spreading Omicron variant will develop mysterious symptoms. Signs of long COVID usually appear weeks after first getting sick. But some experts think a wave of long COVID is likely. And they say doctors need to be prepared for it.

The U.S. National Institutes of Health is carrying out research on the condition, with the help of the $1 billion it received from Congress. Medical centers for studying long COVID are appearing around the world. Such centers have connections with places like Stanford University in California and University College London.

Why does it happen?

Scientists have different theories for why long COVID happens. Support is building around a few of the major theories.

One theory is that a small amount of the virus stays in the body long past the illness. This causes inflammation that leads to long COVID.

Another theory says that inactive viruses in the body are reactivated. A recent study that appeared in the publication Cell pointed to Epstein-Barr in the blood as one of four possible risk factors. Epstein-Barr is a virus that causes mononucleosis – a disease that makes people very tired and weak for a long time.

The study's findings must be confirmed by more research.

A third theory is that autoimmune reactions develop after severe COVID-19. In a normal immune reaction, viral infections activate antibodies that fight virus proteins. But sometimes, antibodies mistakenly attack normal cells. This action is thought to play a part in autoimmune diseases such as lupus and multiple sclerosis.

Another possibility is that very small blood clots in the blood play a part in long COVID. Many COVID-19 patients develop high levels of inflammatory molecules that can lead to abnormal clotting.

In her laboratory at Stellenbosch University in South Africa, scientist Resia Pretorius has found small clots in blood samples from patients with COVID-19 and in those who later developed long COVID. She also found high levels of protein in blood plasma that prevented the normal breakdown of these clots.

Pretorius believes that these clotting abnormalities continue in many patients after a coronavirus infection. She also believes they reduce oxygen going to cells and tissue throughout the body. This lack of oxygen can cause many of the symptoms linked to long COVID.

It can affect nearly anyone

A full list of symptoms that make up long COVID does not exist. The most common ones are tiredness, problems with memory and thinking, loss of taste and smell. Also included are shortness of breath, sleep problems and mental health issues.

Long COVID affects adults of all ages as well as children. Research shows it is more common among those who were hospitalized, but it also affects those who were not. It is more common among women.

Jacki Graham's experience with COVID-19 at the beginning of the pandemic was not bad enough to put her in the hospital. But months later, the 64-year-old experienced breathlessness and a racing heart. She could not taste or smell. Her blood pressure increased.

In the fall of 2020, she started becoming so tired that her morning exercise would send her back to bed.

"Six months ago, I would have told you COVID has ruined my life," Graham said.

Hogan, the New York nurse, also did not go to the hospital with COVID-19. But she has struggled since the infection.

Hogan's doctors think autoimmune problems and a pre-existing connective tissue disorder may have made her likely to develop the condition.

Possible answers

There are no treatments approved for long COVID. Some patients, however, see improvements from painkillers, drugs used for other conditions, and physical therapy. But more help may come soon.

Immunobiologist Akiko Iwasaki is studying the possibility that COVID-19 vaccination might reduce long COVID symptoms. Her team at Yale University in Connecticut is working with a patient group called Survivor Corps. Their research involves vaccinating unvaccinated long COVID patients as a possible treatment.

Two recently released studies offer early evidence that being vaccinated before getting COVID-19 could help prevent long COVID, or at least reduce its severity. Both studies were done before the Omicron version of the new coronavirus appeared.

I'm Ashley Thompson.

And I'm Mario Ritter, Jr.

The Associated Press reported this story. Ashley Thompson adapted it for VOA Learning English.