Extreme fatigue, memory problems, dizziness, shortness of breath, these are just a few of the symptoms linked to long COVID.

Four years after people had to put on masks, isolate and got the shot, researchers are still looking for answers as to why some people still suffer from COVID symptoms while others don't.

"We still have very little answers about what causes long COVID and persistent symptoms in most people," Northwestern Medicine Pulmonary and Critical Care Specialist Marc Sala said.

The CDC reports nearly one in five Americans who had COVID-19 still struggle with long COVID, but interestingly, older adults are less likely to get long COVID.

Nearly three times as many adults, 50 to 59 years old, currently have long COVID than those 80 and older. Sala's research focuses on finding out what's happening.

"What they all had in common is that many of them had these CT scans, which showed that their lungs had inflammation and some amount of scar on them," Sala said.

Sala used advanced gene expression technology and CT scans and found that harmful immune cells continued to infiltrate the lungs, causing inflammation.

"Even three months, six months, nine months or up to a year, we had individuals with an ongoing migration of inflammatory cells," Sala said.

Sala believes that this research will not only lead to new treatments of long COVID, but help people with other lung problems.

"What's next in our research is finding out how to stop the influx of these particular cells that we know to be associated with fibrosis, and trying to help that situation, whether it's after COVID, after flu, or in other conditions," Sala said.

Also, interestingly, the CDC reports the percentage of adults with current long COVID symptoms varies by state.

Kentucky, Alabama, Tennessee and South Dakota have the highest rates, while Hawaii, Maryland and Virginia have the lowest.