The transmission of respiratory pathogens such as the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) can occur by direct (person-to-person) or indirect contact (through contaminated surfaces and fomites) and by airborne transmission. Aerosols and droplets play a crucial role in transmitting pathogens from an infected individual to a susceptible host, carrying the virus in large droplets (up to ~500 m) and small respirable aerosol particles (<5 m diameter) from expiratory events including sneezing, coughing, talking and even breathing. The transport of aerosols and droplets occurs over a distance determined by the interplay of forward momentum in an exhaled jet, droplet sedimentation and evaporation. Beginning with Wells, and subsequently investigated by numerous authors, water droplets of order 100 m diameter are predicted to sediment to the ground over a distance of 1-1.5 m from the average height of an adult before they completely evaporate, providing the rationale for guidelines on physical distancing. Water from smaller droplets can evaporate fully before sedimentation can occur, forming aerosol nuclei from any remaining involatile components which remain airborne for many minutes to hours.