Some viruses are highly contagious (like measles), while other viruses are less so. It’s important to know this in order to better understand the risk associated with the Wuhan virus. The reproduction number R0 [pronounced “R naught”] is a mathematics term that indicates how contagious an infectious disease is. If R0 is more than 1, each existing infection causes more than one new infection. The R0 value of the 1918 flu pandemic was estimated to be between 1.4 and 2.8, while Ebola currently has an R0 of about 1.5 to 2. SARS and HIV have an R0 of about 4, while Measles has an R0 of 12-18, meaning a person who has measles will transmit it to an average of a dozen to a dozen and a half other people.
The R0 for the Wuhan novel Coronavirus was initially estimated in the range 1.4 to 2.6 (lower than the 3.8 initial reports). "We estimate that, on average, each case infected 2.6 (uncertainty range: 1.5-3.5) other people up to January 18, 2020, based on an analysis combining our past estimates of the size of the outbreak in Wuhan with computational modeling of potential epidemic trajectories,” according to a report by scientists at Imperial College London released 25 January 2020. By other reputable estimates, the range looks like 2 to 3.8.