The technical meaning of the word epidemic is an “excess of cases of a disease over the number expected in a given population.” This is an important concept for a physician who may be required to report any unusual disease or group expression of disease to the health department. Influenza normally infects large numbers of people every winter. A few cases of influenza may herald the beginning of the season, but they are expected. Public health reports on the epidemic may even use the term “excess deaths.” Only when a substantial number of people are ill and medical resources are strained does it become an epidemic. In contrast, one or two cases of a rare disease may constitute an epidemic. The occurrence of a few cases of diphtheria anywhere in the United States is an epidemic because we do not expect any cases of this disease.

To most people, the word plague brings to mind images of the decimation of Europe by the Black Death in the Middle Ages. Plague is not a technical term like epidemic. It is generally used to describe epidemic disease that is perceived as a disaster for a community or a specific group. A plague of locusts may be a disaster, but it is not an epidemic. The potential for disease epidemics that qualify as plagues is ever present. Many of these diseases appear and disappear without warning or in cycles that are poorly understood. The sudden appearance of HIV infection is not unusual for a plague. Bubonic plague goes through cycles that last about 400 years; on this cycle, we are due for another worldwide epidemic of bubonic plague. The population of rodents and fleas that is necessary to fuel such an epidemic is present, and the disease is endemic in most of the western United States. The question is not what would start such an epidemic but why it has not already started.

Public health procedures backed by strong laws are necessary to combat plagues. If disease control measures are postponed endlessly while policy is debated, the disease may spread so widely that no measure can contain it. More vigorous public health efforts, such as closing gay bathhouses as soon as it became obvious how HIV was spread, might have reduced the extent of the epidemic. Had the bathhouses been closed in the late 1970s when it became obvious that they were the vector for the spread of hepatitis B (HBV), HIV might have emerged slowly enough that gay men could have learned of its existence before infection became so widespread.