The amount of SARS-CoV-2 virus that needs to be present in the body to trigger the infection is unclear. Other respiratory viruses could offer insight into this number. For example, SARS, another coronavirus, requires just a few hundred viral particles for an infective dose, while the dose for MERS is several thousands of particles.
Much of the data collected on transmission of SARS-CoV-2 has been inconsistent. Patients who are asymptomatic have been found to have viral loads equal to patients who are critically ill. The phenomenon of "super-spreaders" who can pass the virus to many people could provide a clue, but it is unclear yet whether these cases are due to the person's biology or behavior.
Factors like nostril shape, presence of nose hair and mucus, and even the distribution of cellular receptors in the airway can all influence how much virus needs to be received to cause infection. The dose may also vary depending on whether the particles are ingested or inhaled. The Centers for Disease Control and Prevention has publicly said that it does not believe ingestion is the major way the virus is spreading, but rather via aerosol transmission.
Sneezing, coughing, and talking can spread thousands of respiratory droplets carrying the virus into the air. Larger droplets fall quickly due to their weight and will not penetrate surgical masks, but droplets less than 5 microns in diameter, called aerosols, can remain in the air for hours.
Proximity to the infected person, air flow, and timing all seem to be critical factors for aerosol transmission. Using a spray nozzle designed to simulate expulsion of saliva droplets, Dutch researchers found that opening windows or doors introduced enough air flow to expel present aerosols. Data published in the journal Nature from hospitals in Wuhan, China found more aerosolized particles in unventilated restrooms than in ventilated patient rooms or even crowded public areas. Researchers noted that aerosols, due to their size, would contain a lower quantity of the virus than much larger droplets.
Experts agree that in addition to avoiding unventilated and crowded indoor areas, the most effective way to avoid infection is to wear a mask. Even if the mask does not fully shield from respiratory droplets containing the virus, it can help keep the amount of virus the wearer receives below the infective dose.