The novel coronavirus disease (COVID-19) spreads mainly through droplets of saliva or discharge from the nose when an infected person coughs, sneezes, or even talks. The virus may also spread through airborne transmission, although research on this mode of transmission is ongoing.
What makes COVID-19 particularly dangerous is that it can spread through asymptomatic carriers: people who have the virus but show no symptoms. Any infected person without a mask, whether or not they show signs of the virus, risks spreading virus-containing droplets into the environment. When a healthy person breathes in these droplets or touches an infected surface and then their eyes, nose, or mouth, they may become infected.
Masks block a significant portion of infection-carrying droplets from entering the environment, slowing the spread of COVID-19.
Virus spread is understood through the effective reproduction number (Re), which is the number of people in a population who can be infected by an individual at any specific time (CEBM). When Re falls below 1.0, the spread of the virus declines as each infected person infects fewer people. Re may be reduced due to growing immunity in a population or deaths of infected individuals, but it can also be brought down by precautions like social distancing and masks.
An international review of the scientific research on masks concluded that near-universal adoption of non-medical masks in public (in conjunction with other measures like test & trace) can reduce Re below 1.0 and stop the community spread of the virus. Thus, masks save lives by reducing case numbers and preventing large burdens on our healthcare systems.
There are broadly three kinds of masks: N-95, surgical, and cloth masks (in order of highest to least protection offered to the wearer). Healthcare professionals are the most exposed to COVID-19 and therefore require N-95 masks or, at the very least, surgical masks. The COVID-19 pandemic is causing a shortage of these masks, so they should be reserved for healthcare professionals, while everyone else can make their own cloth mask. Wearing homemade cloth masks can still significantly reduce the spread of the virus.
Governments and organizations around the world are recognizing the importance of masks. Over 100 countries have recommended or made masks mandatory in public.