In March 2020, Hannah Davis fell ill, and everything changed. Her respiratory symptoms were mild, but the neurological and cognitive fallout was frightening.

“I could tell very early on that something was wrong with my brain,” she said after getting sick with covid-19.

And Davis had quantitative proof — her score for processing speed on a cognitive test dropped from the 96th percentile right before the pandemic to the 14th percentile after her coronavirus infection.

Davis had been a prolific creator of art and music using machine learning. “I used to be someone who is thinking all the time, just had a constant flow of ideas,” she said. “And that’s entirely gone.”

Five years later, Davis is one of an estimated 20 million Americans and 400 million people worldwide who have had long covid, in which symptoms persist or newly develop more than three months after a coronavirus infection.

Neurocognitive symptoms of long covid are among the most common, affecting 18 to 36 percent of patients — and among the most devastating. Many people have reported cognitive disabilities, with some having to stop working.