In epidemiology, an R0 is a measure of transmission. It basically represents the average number of people that one infected person will, in turn, infect. Based on current information, coronavirus has an R0 somewhere between 2 and 3, so the average infected person will pass the virus on to two or three more people. That makes it more contagious than the flu, which has an R0 of about 1.5. Measles, one of the most contagious viruses, has an R0 of about 15.