California’s state university system, the largest in the United States, canceled classes on Tuesday for the fall semester because of the coronavirus, while Los Angeles County said its stay-at-home order was likely to be extended by three months. The announcements on the West Coast came after the nation’s top infectious disease expert, Dr. Anthony Fauci, told Congress that lifting the sweeping lockdowns could touch off new outbreaks of the illness, which has killed nearly 81,000 Americans and devastated the economy. In one of the first indications the pandemic will continue to have a significant impact into autumn, the chancellor of California State University said classes at its 23 campuses would be canceled for the semester that begins in September, with instruction moved online.