An epidemic is the rapid propagation of a disease, generally contagious and infectious in origin, within a given population. The most notorious historical case of an epidemic is the "Spanish flu," or 1918 flu pandemic, which resulted in over 30 million deaths. The most frequent epidemic diseases today are the seasonal flu, gastroenteritis, and varicella, or chickenpox
. For each disease, the epidemic threshold is fixed at a certain number of cases in 100,000 residents (i.e.: 80 cases in 100,000 residents for the flu).