A disease’s "basic reproduction number" or R0 (pronounced "R naught")
is the average number of people that one person with the disease can infect. As an infection spreads to new people, it reproduces itself. Scientists, therefore, use a disease’s R0 to measure how "contagious" it is.
The R0 is calculated on the assumption that no one in a given population has been vaccinated and no one has ever had the disease
If R0 < 1, then each existing infection causes less than one new infection. In this case, the disease will eventually die out.
If R0 = 1, then each existing infection causes one new infection. In this case, the disease will stay alive and stable, but there won’t be an outbreak or an epidemic.
If R0 > 1, then each existing infection causes more than one new infection. In this case, the disease will spread between people, and there could be an outbreak or epidemic.