A leading explanation for long-Covid symptoms is that immune-system activity and ensuing inflammation continue to affect organs or the nervous system even after the virus is gone, researchers said.

Some of the most compelling evidence for the inflammation theory comes from Covid-19 patients with signs of heart inflammation and injury months after illness. One study looking at 100 Covid-19 patients two months after getting sick found that 78 had abnormal findings on cardiac magnetic resonance imaging, while 60 had cardiac MRIs indicating heart-muscle inflammation. The study included hospitalized, nonhospitalized and asymptomatic patients.

“Even those who had no symptoms and were young and fit…even in those patients we saw abnormalities,” said Eike Nagel, one of the lead authors and director of the Institute for Experimental and Translational Cardiovascular Imaging at the University Hospital Frankfurt in Germany.

Some patients had scarring on their heart imaging, he said, which worried him. The scarring wasn’t too serious, he said, but “we know from other studies that this is related to worse outcomes.”

Doctors also are reporting cases of long-Covid patients with gastrointestinal issues.