Starting Friday, workers and visitors at health care facilities in parts of the Bay Area will be required to wear masks to prevent the spread of COVID and other infectious diseases. The public health departments in Alameda, Contra Costa, Santa Clara, San Mateo and Napa counties have issued mask mandates for hospitals, skilled nursing facilities and other health care facilities. UCSF infectious disease specialist Dr. Peter Chin-Hong pointed out that many doctors and nurses already wear masks. “There might be some people who worry that this is taking away their agency,” he said. “But again, the focus is not on your average person in the community. The focus is really on protecting people who are vulnerable.” The mask mandate takes effect Friday and remains in place through the end of March or April.