The English version was updated on 20 October 2020
COVID-19 is caused by the SARS-CoV-2 virus, which spreads between people, mainly when an infected person is in close contact with another person.
The virus can spread from an infected person’s mouth or nose in small liquid particles when they cough, sneeze, speak, sing or breathe heavily. These liquid particles are different sizes, ranging from larger ‘respiratory droplets’ to smaller ‘aerosols’.
Other people can catch COVID-19 when the virus gets into their mouth, nose or eyes, which is more likely to happen when people are in direct or close contact (less than 1 metre apart) with an infected person.
Current evidence suggests that the main way the virus spreads is by respiratory droplets among people who are in close contact with each other.
Aerosol transmission can occur in specific settings, particularly in indoor, crowded and inadequately ventilated spaces, where infected person(s) spend long periods of time with others, such as restaurants, choir practices, fitness classes, nightclubs, offices and/or places of worship. More studies are underway to better understand the conditions in which aerosol transmission is occurring outside of medical facilities where specific medical procedures, called aerosol generating procedures, are conducted.
The virus can also spread after infected people sneeze, cough on, or touch surfaces, or objects, such as tables, doorknobs and handrails. Other people may become infected by touching these contaminated surfaces, then touching their eyes, noses or mouths without having cleaned their hands first.
Find out more about the science about how the SARS-CoV-2 virus infects the body and how our body’s immune system reacts by watching or reading this interview with WHO’s technical lead for COVID-19, Dr Maria Van Kerkhove.
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.
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.
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 or no ventilation, are riskier than outdoor locations.
Transmission can occur more easily in the “Three C’s”:
The risk of COVID-19 spreading is higher in places where these “3Cs” overlap.
In health facilities, some medical procedures, called aerosol generating procedures, can produce very small droplets (called ‘droplet nuclei’ or ‘aerosols’) that can stay suspended in the air for longer periods of time. This is why health workers performing these procedures take specific airborne protection measures, including using appropriate personal protective equipment, including respirators, and why visitors are not permitted in areas in which these procedures are being performed.
To limit the risks of getting COVID-19 follow these basic precautions:
Read our public advice page for more information.