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Viruses spread between people in respiratory droplets of various sizes exhaled while breathing, talking, singing, laughing, coughing and sneezing. Once exhaled there are 4 main categories of transmission:

  • Direct contact - Hand to hand contact with an infected person followed by hand to eyes, nose and mouth contact. 

  • Indirect contact - Hand to infected surface contact followed by hand to eyes, nose and mouth contact.

  • Large droplets - Droplets larger than 0.5 microns projected in the breath, cough or sneeze of an infected person directly into the eyes, nose or mouth of another person.

  • Aerosols - Droplets of less than 0.5 microns emitted by an infected person that float in the air and travel in air currents. Particles of this size can be inhaled deep into the lungs of another person.

There are many factors that influence how the transmission occurs and scientific research is ongoing into the important questions for which we need better answers. Fortunately, these viruses are physically very fragile and can be mitigated by self-isolation, physical distancing, handwashing, face masks, indoor air ventilation and filtration, as well as by cleaning and sanitizing. 

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The virus reponsible for


COVID-19 Disease




Diameter of a SARS-CoV-2 Virus

Average = 125 nanometers (0.125 microns)

Range = 60 to 140 nanometers (0.06 to 0.140 microns)

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The virus responsible for flu

Influenza A, B, and C Viruses

CDC-inluenza virus.jpg
CDC-H1N1 influenza virus.jpg

Diameter of an Influenza Virus

Average = 100 nanometers (0.10 microns)

Range = 80 to 120 nanometers (0.08 to 0.120 microns)

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Why do the virus and the disease have different names? 

Viruses, and the diseases they cause, often have different names.  For example, HIV is the virus that causes AIDS. The name of a disease is often better known than the name of the virus that causes it. In science and disease control there are different processes, and purposes, for naming viruses and diseases.

Viruses are named based on their genetic structure to facilitate the development of diagnostic tests, vaccines and medicines. Virologists and the wider scientific community do this work, so viruses are named by the International Committee on Taxonomy of Viruses (ICTV).


Diseases are named to enable discussion on disease prevention, spread, transmissibility, severity and treatment. Human disease preparedness and response is the WHO’s role, so diseases are officially named by the WHO in the International Classification of Diseases (ICD).

ICTV announced “severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)” as the name of the new virus on 11 February 2020.  This name was chosen because the virus is genetically related to the coronavirus responsible for the SARS outbreak of 2003.  While related, the two viruses are different.


The WHO announced “COVID-19” as the name of this new disease on 11 February 2020, following guidelines previously developed with the World Organisation for Animal Health (OIE) and the Food and Agriculture Organization of the United Nations (FAO). Source: World Health Organization, Coronavirus Disease (COVID-19), Technical Guidance, 2020


Data-based  prevention and control of COVID-19

A Scientific Brief from the WHO

The transmission of SARS-CoV-2:

Implications for prevention and control  precautions. 

"To the best of our understanding, the virus is primarily spead through contact and respiratory droplets. Under some circumstances airborne transmission may occur, such as when aerosol generating procedures are conducted in health care settings, or potentially in indoor crowded poorly ventilated settings elsewhere." ~ The World Health Organization, based on their ongoing careful review of the emerging scientific evidence.  

To prevent transmission, the WHO recommends a comprehensive set of measures including (abridged):

  • At all times practice frequent hand hygiene; 

  • Physical distancing from others when possible; 

  • Respiratory etiquette; 

  • Avoid crowded places and enclosed spaces spaces with poor ventilation;

  • Wear fabric masks when in enclosed, overcrowded spaces to protect others;

  • Ensure good environmental ventilation in all closed settings;

  • Appropriate environmental cleaning and disinfection.  

Source: World Health Organization Scientific Brief, Transmission of SARS-CoV-2: implications for infection prevention precautions, 9 July 2020.

How to prevent transmission

Main findings

  • Understanding how, when and in what types of settings SARS-CoV-2 spreads between people is critical to develop effective public health and infection prevention measures to break chains of transmission.

  • Current evidence suggests that transmission of SARS-CoV-2 occurs primarily between people through direct, indirect, or close contact with infected people through infected secretions such as saliva and respiratory secretions, or through their respiratory droplets, which are expelled when an infected person coughs, sneezes, talks or sings.

  • Airborne transmission of the virus can occur in health care settings where specific medical procedures called aerosol generating procedures, generate very small droplets called aerosols. Some outbreak reports related to indoor crowded spaces have suggested the possibility of aerosol transmission, combined with droplet transmission ....

  • Respiratory droplets from infected individuals can also land on objects, creating fomites (contaminated surfaces). As environmental contamination has been documented by many reports, it is likely that people can also be infected by touching these surfaces and touching their eyes, nose or mouth before cleaning their hands.

  • Based on what we currently know, transmission of SARS-CoV-2 is primarily occurring from people when they have symptoms, and can also occur just before they develop symptoms, when in close proximity to others for prolonged periods of time. While someone who never develops symptoms can also pass the virus to others, it is still not clear to what extent this occurs. More research is needed in this area.

  • High quality research is urgently needed to elucidate the relative importance of different transmission routes; the role of airborne transmission in the absence of aerosol generating procedures; the dose of virus required for transmission to occur; the settings and risk factors for superspreading events; and the extent of asymptomatic and pre-symptomatic transmission.

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Coronavirus (COVID-19) 

Guidance from the

Canadian Centre for Occupational Health and Safety (CCOHS)


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Transmission of Viruses in Droplets and Aerosols

Part 1

Part 2

Part 3

Part 4

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