The "epidemic threshold" refers to the critical point at which the number of infections in a population becomes sufficient to sustain an outbreak. Below this threshold, the disease tends to die out, while above it, the infection can spread rapidly. This concept is important in understanding how diseases like influenza or COVID-19 can transition from isolated cases to widespread outbreaks. Factors influencing the epidemic threshold include the rate of transmission, the duration of infectiousness, and the population's immunity levels. Public health measures, such as vaccination and social distancing, can help lower the number of infections and keep the population below the epidemic threshold, thereby preventing large-scale outbreaks.