Scientists say the 'dose' of coronavirus a person is infected with may determine how ill they become.
This means contracting a very low amount of the virus may result in being asymptomatic with no coughing, fever or shortness of breath.
However, a modest dose may result in someone have a mild-to-moderate infection and a very high dose could lead to hospitalization, or even death.
Dr Erin Bromage, an associate professor of biology at the University of Massachusetts Dartmouth, told CNN there is no certain number of viral particles needed to fall ill - but the more you have increases the odds that at least one will enter and infect cells, thus setting off a series of events.
Experts say the amount of coronavirus a person is infected may determine how sick they become with a low 'dose' resulting in asymptomatic illness but a high dose leading to hospitalization. Pictured: Medical personnel attend to a patient at Bellevue Hospital in New York, October 28
Because masks reduce the amount of particles inhaled, two doctors say face coverings could be a way to generate immunity until there is a vaccine. Pictured: People wearing masks and gloves wait to enter a Walmart in Uniondale, New York, April 17
'If you hit an animal with a low enough dose, they'll be able to fend that off without developing any disease at all,' he told CNN.
'If you get a magic number of an infectious dose, an infection will establish and that animal will then succumb to the disease from that particular pathogen.
'But if you hit them with more than the infectious dose...like a high dose of a virus, for example, leads to more severe outcomes. So, dose becomes really important.'
Bromage adds that a person may not be infected with a high dose all at once but it could occur over multiple days.
This is line with the Centers for Disease Control and Prevention (CDC) redefining 'close contact' last month to include several brief exposures with