As many as 1 in 4 people who are infected with COVID-19 may not have symptoms, according to CDC director Robert Redfield, MD. The high rate of asymptomatic cases is prompting the agency to consider expanding its guidelines on who should wear masks. "This helps explain how rapidly this virus continues to spread across the country,” Redfield said in an NPR interview on Tuesday. Although CDC has said that the public does not need to wear masks unless they are feeling sick, with the new data on people who may be infected without showing any symptoms, Redfield the guidance was "being critically re-reviewed." It is not known exactly how many people are infected without feeling sick, or is some of them are presymptomatic. Experts including Michael T. Osterholm, MD, an infectious disease expert at the University of Minnesota, note that with any vaccine still in early development, the best way to stem the spread of the disease is social distancing. Furthermore, because people may be spreading the virus to others even when they feel healthy, asking only sick people to stay home is not likely to be sufficient—which is why many experts are now encouraging everyone to wear masks, to prevent those who do not know they are infected from spreading it.