Basic reproduction number
A very important number for describing whether a disease can become an epidemic or not is R0, pronounced "R naught" or "R zero". It refers to how many people a person who has this disease is expected to infect on average. It is an abbreviation for basic reproduction number.
If R0 > 1, a disease can become an epidemic. If R0 < 1, it cannot. Many known diseases have R0 < 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 (Re, usually written Rt with t for time) lower than the basic reproduction rate (R0).
An example of a vaccine that works really well is the smallpox vaccine, which stopped smallpox virus from spreading so well that it no longer exists except in laboratories. Examples of making the effective reproduction rate lower than the basic reproduction rate are using condoms to stop sexually transmitted diseases from spreading or not getting close to others (physical distancing, often called social distancing) to stop respiratory diseases from spreading.