Whenever there’s a new outbreak, scientists rush to calculate a number called R0, or R-naught. Why? It’s been a critical part of the scientific effort to understand just how transmissible the new virus is. Here’s how.

Editor’s Note (2/11/20): The video in this story has been revised after posting. It originally incorrectly referred to SARS as “sudden acute respiratory syndrome.” The full name is “severe acute respiratory syndrome.”