A key quantity in epidemiology is : the basic reproduction number (which you may have heard about in the news). , is the number of people that a given person is likely to infect. A good diagram of this is shown below for Ebola with and the 2002 SARS outbreak with . Given that the number of people to whom an infected person is likely to spread the disease will depend on mitigation measures, will change over the course of the outbreak. A disease with will continue to spread (the number of people continues to grow), meanwhile results in the number of cases declining. Given that is just a number of people, it is a unit-less number.
Measure for different stages of the COVID-19 pandemic in the United States during 2020.