It causes neurological issues, possibly linked to the immune overreaction it creates. There will be people who become permanently ill because of this. From this article in The Atlantic
When I spoke with [so-called "recovered" patient] LeClerc on day 66, she was still experiencing waves of symptoms. “Before this, I was a fit, healthy 32-year-old,” she said. “Now I’ve been reduced to not being able to stand up in the shower without feeling fatigued. I’ve tried going to the supermarket and I’m in bed for days afterwards. It’s like nothing I’ve ever experienced before” ... Today is day 80.
Most [long-haulers] have never been admitted to an ICU or gone on a ventilator, so their cases technically count as “mild.” But their lives have nonetheless been flattened by relentless and rolling waves of symptoms that make it hard to concentrate, exercise, or perform simple physical tasks. Most are young. Most were previously fit and healthy. “It is mild relative to dying in a hospital, but this virus has ruined my life,” LeClerc said. “Even reading a book is challenging and exhausting.”
Even though the world is consumed by concern over COVID-19, the long-haulers have been largely left out of the narrative and excluded from the figures that define the pandemic. I can pull up an online dashboard that reveals the numbers of confirmed cases, hospitalizations, deaths, and recoveries—but LeClerc falls into none of those categories. She and others are trapped in a statistical limbo, uncounted and thus overlooked.
[M]any long-haulers have faced disbelief from friends and medical professionals because they don’t conform to the typical profile of the disease. People have questioned how they could possibly be so sick for so long, or whether they’re just stressed or anxious. “It feels like no one understands,” said Chloe Kaplan from Washington, D.C., who works in education and is on day 78.
[N]eurological symptoms are common among the long-haulers ... As many people reported “brain fogs” and concentration challenges as coughs or fevers. Some have experienced hallucinations, delirium, short-term memory loss, or strange vibrating sensations when they touch surfaces. Others are likely having problems with their sympathetic nervous system, which controls unconscious processes like heartbeats and breathing: They’ll be out of breath even when their oxygen level is normal, or experience what feel like heart attacks even though EKG readings and chest X-rays are clear. These symptoms wax, wane, and warp over time.