If R0 &gt; 1, a disease can become an epidemic. If R0 &lt; 1, it cannot. Most commonly known diseases have R0 &gt; 1. However, vaccines can be used to make enough people immune in a population to stop epidemics from happening. We can also use other measures to make the effective reproduction rate (Re, usually written Rt with "t" for time) lower than the basic reproduction rate (R0).