Incidence refers to the number of individuals who develop a specific disease or illness (like COVID-19) during a given period of time, according to the CDC. The incidence rate is the number of new cases of a disease existing in a population at a specific time divided by the total number of people at risk for that disease in that population. Monitoring changes in incidence in different areas is important for experts to understand community spread and what types of social distancing measures need to be in place.