Growing evidence suggests that SARS-CoV-2, the novel coronavirus that causes COVID-19, can be spread by asymptomatic people via aerosols -- a reality that deeply underscores the ongoing importance of regular widespread testing, wearing masks and physical distancing to reduce the spread of the virus, say Kimberly Prather and colleagues in a new Perspective. The World Health Organization (WHO) recommendations for social distancing of 6 feet and hand washing to reduce the spread of SARS-CoV-2 are based on studies of respiratory droplets carried out in the 1930s. When these studies were conducted, the technology did not exist for detecting submicron aerosols. More recently, measurements show that intense coughs and sneezes that propel larger virus droplets more than 20 feet can also create thousands of aerosols that can travel even further. Recent studies of SARS-CoV-2 have shown that in addition to droplets, SARS-CoV-2 may also be transmitted through aerosols. One recent study estimated that a single minute of loud speaking might generate between 1,000-100,000 virion-containing aerosols or virus particles suspended in the air, for example. These infectious aerosols can accumulate in indoor, uncirculating air for hours, where they can be more easily inhaled deeply into the lungs. Given how little is known about the airborne behavior of infectious aerosols, it's difficult to define a safe distance for physical distancing, the authors say. Properly fitted masks provide a critical physical barrier, reducing the number of infectious viruses in the exhaled breath of asymptomatic individuals. Countries that have been most effective in limiting the spread of COVID-19 have implemented universal masking, they note. It is particularly important to wear masks in locations with conditions that can accumulate high concentrations of viruses, such as health care settings, airplanes, restaurants, and other crowded places with reduced ventilation.