One fundamental question about any novel pathogen is: how does it transmit? Answering this question will help to protect ourselves from the agent, at least until effective vaccines and antiviral therapies can be developed, especially if it is an agent of moderate to high lethality. Initially, at the start of the Coronavirus disease 2019 (COVID-19) pandemic, more emphasis was placed on handwashing rather than on droplet and aerosol transmission. Although severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2)-infected secretions such as saliva can spread the virus to hands, it became increasing evident that the virus mostly transmitted through close contact (though not necessarily touching), whilst people were breathing, talking, laughing, singing, coughing and sneezing near one another. During such respiratory activities, droplets and aerosols are produced together, and the amount of transmission due to these different-sized liquid particles will likely vary between individuals at different stages of their infection and illness. This question became even more complex as it emerged that viral transmission can occur for several days before symptom onset, and that asymptomatic cases can also shed just as much virus and potentially transmit it just as well as symptomatic cases. This chapter summarizes our understanding of how SARS-CoV-2 transmits and the infection control precautions to reduce this.
