The concept of needing a certain dose of a pathogen — a disease-causing organism — to trigger an infection has been shown to be the case for many viruses, such as the influenza virus, poxviruses and others, explained Erin Bromage, an associate professor of biology at the University of Massachusetts, Dartmouth.

“If you hit an animal with a low enough dose, they’ll be able to fend that off without developing any disease at all. 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, in most situations a high dose of pathogens — like a high dose of a virus, for example — leads to more severe outcomes. So, dose becomes really important,” he said, calling the relationship “dose-dependent.”

It’s not because you need a certain number of particles of virus to infect a cell — it just increases the odds that one of those viral particles will make it into the cell and infect it, setting off the chain reaction.