If R > 1, a disease can become an epidemic. If R < 1, it cannot. Many known diseases have R < 1 and cannot become epidemics. If they occur only occasionally, they are called sporadic. When there is a steady level of regular cases occurring in a region, the disease is called endemic. Doctors can use vaccines 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 (R, usually written R with "t" for time) lower than the basic reproduction rate (R).