An epidemic describes the rapid spread of a virulent contagious disease throughout a population. It should not be used interchangeably with the term pandemic, which is the outbreak of any disease within a population for any reason, including contagion. For example, an epidemic of influenza is a fairly common occurrence. However, the incidence of diabetes mellitus keeps growing, it cannot be described as an epidemic because it is not passed from person to person. The study of the pattern of epidemics is one of the most important parts of epidemiology, although epidemiology studies the spread of any sort of disease, often to determine if the disease is contagious or whether it is more likely to occur in certain environments.
Epidemics usually strike a population that has no natural immunity to a disease, and are the most common with influenza, where the strains change so fast that entire populations are likely not to be immune to the disease. They are often characterized by very few cases growing to a large number of cases in a very short time, only to have the number of ill people collapse either as patients recover or die.
Small epidemics are still common, and one of the main purposes of the Centers for Disease Control in Atlanta is to deal with outbreaks of contagious diseases to prevent their further spread. Many diseases, such as smallpox, were largely eradicated using such methods.
Diseases that are not terribly contagious rarely cause epidemics unless they are very slow to act. For example, it could be argued that the epidemic of AIDS was the result of its slow action despite the difficulty in transmitting the disease. However, the spread of AIDS is not typical of many epidemics as it only struck those who engaged in high risk behaviors for the disease.
Influenza is still the disease most likely to cause an epidemic. The last great worldwide epidemic was one of influenza in 1918-1919 that killed over 20,000,000 people - more people than had been killed in World War I.
Epidemics were far more common in the past. Plague is estimated to have killed about half the population of Europe in the Middle Ages. Smallpox is estimated to have killed up to 95% percent of native Americans from the 15th to 19th centuries.
Small populations are often very susceptible to epidemics, because it is less likely that members of a small population will be exposed to the disease on a regular basis. Instead, everyone is exposed all at once. For example, the Faroe Islands, a small island of about 70,000 people of Danish descent in the North Sea, either has no cases of measles or just about everyone with measles as a population this small will not support measles for long - the disease needs about 500,000 people in order to be constantly present in the population.