https://www.mercurynews.com/2020/02/28/santa-clara-county-announces-new-coronavirus-case/

Santa Clara County health officials Friday announced their third case of the novel coronavirus that has killed thousands and alarmed health officials and investors around the world.

More troubling, sources said it is unknown how the person became infected, marking only the second U.S. case in which the source of infection is unknown, suggesting the virus may be circulating in the community.