IPA: /ˌɛpɪˈdɛmɪsɪti/
KK: /ɛpɪˈdɛmɪsɪti/
The state or condition in which a disease spreads rapidly and widely among a population.
The epidemicity of the flu this season has caused many people to get vaccinated.
Epidemicity → It is formed from "epi-" (meaning upon or over) and "demos" (meaning people) combined with the suffix "-icity" (indicating a state or condition). The word refers to the state or condition of affecting a large number of people, particularly in the context of disease.
Think of something that affects people 'upon' ('epi-') and relates to 'people' ('demos') — that's why epidemicity describes the condition of widespread impact on a population.