The term that describes a disease affecting a significant portion of the population is "epidemic." An epidemic occurs when the incidence of a disease significantly exceeds what is normally expected within a particular area or population, often leading to widespread community transmission. This concept can apply to infectious diseases, where factors such as the contagiousness of the pathogen and the level of population immunity influence its spread. When epidemics occur on a global scale, they may be termed "pandemics." Public health interventions often aim to control or mitigate epidemics through education, vaccination, and other preventive measures.