Also called the R₀, pronounced "R naught," the term refers to a number that indicates the infectiousness of a virus or a pathogen in a particular location. "For every one person infected, how many others will be infected?" quoting Melissa Nolan. For COVID-19, the R₀ is currently estimated at 2.2, which means that's the average number of expected secondary cases caused by a single infected person.

"Really, it's an indication of how quickly something could spread," she adds.