Yuanyuan Zhu was walking to her gym in San Francisco on March 9, thinking the workout could be her last for a while, when she noticed that a man was shouting at her. He was yelling an expletive about China. Then a bus passed, she recalled, and he screamed after it, “Run them over.” She tried to keep her distance, but when the light changed, she was stuck waiting with him at the crosswalk. She could feel him staring at her. And then, suddenly, she felt it: his saliva hitting her face and her favorite sweater. In shock, Ms. Zhu,...