Epidemic

Epidemic, outbreak of contagious disease affecting an unusually large number of people or involving an extensive geographical area. Epidemics, which may be short-lived or last for years, are brought on by the widening reach of disease-causing organisms. These organisms can spread by contaminated food or water, directly from one person to another through physical contact, or by the exchange of bodily secretions such as saliva, semen, or blood. Insects, rodents, and other disease-carrying animals, known as vectors, are agents that may infect human populations with epidemic diseases.

Among the diseases that have occurred in epidemic proportions throughout history are bubonic plague, influenza, smallpox, typhoid fever, tuberculosis, cholera, bacterial meningitis, and diphtheria. Occasionally, childhood diseases such as mumps and German measles become epidemics.

In the past, when sanitary conditions were poor and diseases were little understood, epidemics occurred periodically and killed thousands of people. One of the largest epidemics ever recorded was the outbreak of bubonic plague that raged throughout Europe, Africa, and Asia from 1347 to 1350. This epidemic, known as the Black Death in Europe, is estimated to have killed one-third of the European population. An outbreak of influenza in 1918 killed more than 20 million people around the world. Such global epidemics are commonly called pandemics.

Wars and foreign invasions have traditionally provided breeding grounds for epidemic disease. Prior to the 20th century, every European war produced more deaths from disease than from the use of weaponry. Colonists arriving in the western hemisphere carried disease-causing organisms to which they were immune but that devastated the populations of Native Americans who had no previous exposure to these organisms. Due to the spread of disease, the population of central Mexico shrank by an estimated 90 percent in the first 50 years of Spanish domination.

Epidemics can often be prevented or controlled by immunization, improved sanitation, and by other public health measures such as the use of pesticides to wipe out disease-carrying insects. During the 1960s and 1970s, the medical profession hoped that epidemic diseases were well on their way to extinction. Poliomyelitis, an infectious viral disease of the central nervous system that had once been a scourge of young people in the United States, no longer appeared in significant numbers, and other diseases, including smallpox, tuberculosis, and cholera seemed almost neutralized.

But since the 1970s, a number of new diseases, including acquired immunodeficiency syndrome (AIDS), Ebola hemorrhagic fever, hepatitis C, and severe acute respiratory syndrome (SARS) have been identified. New drug-resistant strains of influenza, tuberculosis, meningitis, and cholera have also appeared, causing a resurgence in the number of cases of these diseases.

Programs that identify and control disease are now in place through most of the world and have repeatedly shown themselves capable of responding quickly and effectively to sudden outbreaks of disease. In particular, the Centers for Disease Control and Prevention (CDC), an agency of the federal government of the United States, and the World Health Organization (WHO), an agency of the United Nations, are active in monitoring and providing rapid response to potentially epidemic outbreaks (see Epidemiology).