Infectivity begins as early as three days before symptoms appear and people are most infectious just prior to and during the onset of symptoms. It declines after a first week, but infected people remain contagious for up to 20 days. People can spread the disease even if they are asymptomatic.
Infectious particles range in size from aerosols that remain suspended in the air for long periods of time to larger droplets that remain airborne or fall to the ground. Various groups utilize terms such as "airborne" and "droplet" both in technical and general ways leading to confusion around terminology. Additionally, COVID-19 research has defined the traditional understandings of how respiratory viruses are transmitted. The largest droplets of respiratory fluids do not travel far and can be inhaled or land on mucous membranes on the eyes, nose, or mouth to infect. Aerosols are higher in concentration when people are in close proximity, which leads to easier viral transmission when people are physically close, but air borne transmission occur at longer distances, mainly in locations that are poorly ventilated; in those situations particles can remain suspended in the air for minutes to hour.
The number of people generally infected by one infected person varies; as only 10% - 20% people are responsible for the disease's spread. It often spreads in clusters, where infection can be tracked back to an index case or geographical location. Often in these instances, superspreading events occur where people are infected by one person.
