Dozens of health-care workers have fallen ill with covid-19, and more are quarantined after exposure to the virus, an expected but worrisome development as the U.S. health system girds for an anticipated surge in infections. From hotspots such as the Kirkland, Wash., nursing home where nearly four dozen staffers tested positive for the coronavirus, to outbreaks in Massachusetts, Pennsylvania, California and elsewhere, the virus is picking off doctors, nurses and others needed in the rapidly expanding crisis. “We all suspect it’s the tip of the iceberg,” said Liam Yore, a board member of the American College of Emergency Physicians.