Short answers, pulled from the story.
The United States Centers for Disease Control and Prevention defines an epidemic as a sudden increase in disease cases above what is normally expected in a population. This threshold distinguishes true epidemics from normal occurrence or smaller outbreaks.
Antigenic drift arises over time through accumulated mutations in virus genes across multiple hosts, while antigenic shift occurs abruptly when two or more different virus strains coinfect a single host. These processes create new strains capable of evading existing immunity within populations.
The 1854 Broad Street cholera outbreak began when contaminated water supplied a London neighborhood. Removing the pump handle mitigated the epidemic and established foundations for modern epidemiology science.
Thucydides described the Plague of Athens striking in 430 BC, creating one of earliest recorded epidemic accounts. Michiel Sweerts painted The Plague of Athens between 1654 and 1700 to illustrate devastating impacts on the city.
Common source outbreak epidemics include point source outbreaks that develop over single exposure, continuous or intermittent outbreaks with variable exposure, propagated outbreaks spreading person-to-person, and mixed outbreak types combining characteristics of both patterns.