Once a virus infects a host, it replicates within the host’s cells to create copies that can be passed on, or transmitted, to others.
Virus transmission can occur through multiple pathways.
For example, some viruses can travel within the droplets of mucus and spit that are ejected when an infected person breathes, talks, coughs, or sneezes. The virus can be passed on when those respiratory droplets land in the mouth or nose of someone else. Larger respiratory droplets are heavier than air, so gravity pulls them to the ground almost immediately after leaving a person's body. Social distancing recommendations are meant to prevent the spread of a virus through these larger respiratory droplets.
Smaller droplets (below 5 microns, or 5 millionths of a meter; about the size of a red blood cell) are sometimes called aerosols. These smaller, lighter droplets can be suspended in air for longer periods of time, and they can travel longer distances. This can lead to airborne transmission of a virus.
Virus particles emitted from the respiratory tract of an infected individual also can land on surfaces, where they can linger. The virus can spread if someone touches that surface, then touches their own mouth, nose, or eyes. This mode of viral spread is called contact transmission.
The Spread of SARS-CoV-2
The main mode of transmission for SARS-CoV-2 is through exposure to respiratory fluids that contain the virus. Exposure can come from inhaled aerosols (very small respiratory droplets produced when breathing and talking); larger respiratory droplets from coughing or sneezing that are deposited on mucous membranes in the mouth, nose, or eye; and, in rare cases, contact with surfaces that have the virus on them.
The virus can spread even among people who do not exhibit symptoms, which contributes to its rapid propagation.
This is why experts recommend social distancing, wearing face coverings, and frequent hand washing as safeguards against contracting the disease.