Epidemics are outbreaks of disease of bacterial or viral origin that involve many people in a localized area at the same time. An example of an epidemic is the hemorrhagic fever outbreak caused by the Ebola virus in Zaire in 1976. When Ebola fever occurs, it tends to be confined to a localized area, and can involve many people. If an outbreak is worldwide in scope, it is referred to as a pandemic. The periodic outbreaks of influenza can be pandemic.

Some maladies can be both epidemic and pandemic. This can be a function of time. An example is Acquired Immunodeficiency Syndrome (AIDS). Initially, the acknowledged viral agent of AIDS, the Human Immunodeficiency Virus (HIV), was prevalent in a few geographic regions, such as Haiti, and among certain groups, such as homosexual men in the United States. In these regions and populations, the infection was...