“Epidemiologists-scientists who study the spread of disease-use a special number to describe how contagious a virus is. It's called the basic reproduction number, or R0 for short. It's complicated to calculate but simple to understand-it counts how many people one sick person is expected to infect over the course of his or her illness. If I'm sick with a cold and I make two other people sick, the R0 of my virus is 2. Colds and seasonal flus typically have R0 values of around 1.5 to 2. The 1918 flu pandemic R0 was estimated to be 2 to 3, while diseases like polio and small pox have R0 values of around 5 to 7.”