not introduce social distancing until more than two weeks after its first cases. The World Health Organization (WHO) has suggested using the term "physical distancing" instead of "social distancing" because it is physical separation which prevents transmission; people can remain socially connected by meeting outdoors at a safe distance (when there is no stay-at-home order) and by meeting via technology. Definition The American Centers for Disease Control and Prevention (CDC) have described social distancing as a set of "methods for reducing frequency and closeness of contact between people in order to decrease the risk of transmission of disease". During the 2009 swine flu pandemic the WHO described social distancing as "keeping at least an arm's length distance from others, [and] minimizing gatherings". During the COVID-19 pandemic, the CDC defined social distancing as "remaining out of congregate settings, avoiding mass gatherings, and maintaining distance (approximately ) from others when possible". Social distancing, combined with the use of face masks, good respiratory hygiene and hand washing, is considered the most feasible way to reduce or delay a pandemic. Measures Several social distancing measures are used to control the spread of contagious illnesses. Research indicates that measures must be applied rigorously and immediately in order to be effective. Avoiding physical contact Keeping a set physical distance from each other and avoiding hugs and gestures that involve direct physical contact, reduce the risk of becoming infected during outbreaks of infectious respiratory diseases (for example, flu pandemics and the COVID-19 pandemic of 2020.) These distances of separation, in addition to personal hygiene measures, are also recommended at places of work. Where possible, remote work may be encouraged. The distance advised by authorities varies. During the COVID-19 pandemic, for example, the World Health Organization recommends that a distance of or more is safe. Subsequently, China, Denmark, France, Hong Kong, Lithuania and Singapore adopted a 1 m social distancing policy. South Korea adopted . Australia, Belgium, Germany, Greece, Italy, Netherlands, Portugal and Spain adopted . The United States adopted , and Canada adopted . The United Kingdom first advised 2 m, then on July 4, 2020 reduced this to "one metre plus" where other methods of mitigation such as face masks were in use. The WHO's one-metre recommendation stems from research into droplet-based transmission of tuberculosis by William F. Wells, which had found that droplets produced by exhalation, coughs, or sneezes landed an average of from where