We know that the disease is caused by the SARS-CoV-2 virus, which spreads between people in several different ways. The virus can spread from an infected person 's mouth or nose in small liquid particles when they cough, sneeze, speak, sing or breathe. These particles range from larger respiratory droplets to smaller aerosols. Current evidence suggests that the virus spreads mainly between people who are in close contact with each other, typically within 1 metre (short-range). A person can be infected when aerosols or droplets containing the virus are inhaled or come directly into contact with the eyes, nose, or mouth. The virus can also spread in poorly ventilated and/or crowded indoor settings, where people tend to spend longer periods of time. This is because aerosols remain suspended in the air or travel farther than 1 metre (long-range). People may also become infected by touching surfaces that have been contaminated by the virus when touching their eyes, nose or mouth without cleaning their hands. Further research is ongoing to better understand the spread of the virus and which settings are most risky and why. Research is also under way to study virus variants that are emerging and why some are more transmissible. For updated information on SARS-CoV-2 variants, please read the weekly epidemiologic updates. When do infected people transmit the virus? Whether or not they have symptoms, infected people can be contagious and the virus can spread from them to other people. Laboratory data suggests that infected people appear to be most infectious just before they develop symptoms (namely 2 days before they develop symptoms) and early in their illness. People who develop severe disease can be infectious for longer. While someone who never develops symptoms can pass the virus to others, it is still not clear how frequently this occurs and more research is needed in this area. What is the difference between people who are asymptomatic or pre-symptomatic? Don’t they both mean someone without symptoms? Both terms refer to people who do not have symptoms. The difference is that ‘asymptomatic’ refers to people who are infected but never develop any symptoms, while ‘pre-symptomatic’ refers to infected people who have not yet developed symptoms but go on to develop symptoms later. Are there certain settings where COVID-19 can spread more easily? Yes, any situation in which people are in close proximity to one another for long periods of time increases the risk of transmission. Indoor locations, especially settings where there is poor ventilation, are riskier than outdoor locations. Activities where more particles are expelled from the mouth, such as singing or breathing heavily during exercise, also increase the risk of transmission. The “Three C’s” are a useful way to think about this. They describe settings where transmission of the COVID-19 virus spreads more easily: Crowded places; Close-contact settings, especially where people have conversations very near each other; Confined and enclosed spaces with poor ventilation. The risk of COVID-19 spreading is especially high in places where these “3Cs” overlap.