I think we know that this coronavirus is transmitted by respiratory droplets, and primarily through coughing or sneezing, but also through other means, through your mouth and nose primarily. Those are the primary routes of transmissions. We know the tried and true measures to reduce that risk, but we've also recommended non-medical masks—we're preserving the medical ones for health care workers—as an added layer of protection on top of the other public health measures.
On May 19th, 2020. See this statement in context.