In the fall, after Samar Khan came down with a mild case of COVID-19, she expected to recover and return to her previous energetic life in Chicago. After all, she was just 25, and healthy. But weeks later, she said, “this weird constellation of symptoms began to set in.” She had blurred vision encircled with strange halos. She had ringing in her ears, and everything began to smell like cigarettes or Lysol. One leg started to tingle, and her hands would tremble while putting on eyeliner. She also developed “really intense brain fog,” she said. Trying to concentrate on a...