Basic reproduction number
The basic reproduction number (R0), also known as just the reproductive number, basic reproductive number, basic reproductive ratio, reproduction number, R nought or R zero, is a term in epidemiology for the average number of cases in a susceptible population that will be generated by an existing diseased individual carrying an infectious disease. Such a number can be derived empirically or approximated by various mathematical equations and models, but will not necessarily be fixed.
Although immunizations do not affect R0, changes in human behavior and geography do; and it is therefore possible to see a spread of basic reproduction numbers for the same disease that are all correct. Although R0 is not without some complexity and controversy as a concept, it provides a simple way to understand how contagious a disease is. In a population before immunization (which may occur naturally or by vaccination campaigns), a disease with an R0 >1 has the potential to spread, and a disease with a high R0 has the potential to become an epidemic or a pandemic. If the R0 is <1 then a disease usually dies out.
The basic reproductive number should not be confused with the effective reproduction number (R) of a disease. Effective reproduction number is the average number of cases generated in the whole population, not just those who are susceptible.