During the 16th century the Spaniards imposed their culture and religion on Mexico, Central America, and the Inca Empire of Peru. Even with their firearms they could not, in the long run, have had such a significant effect without the aid of disease. The approximately 75 million American Indians whom the Spaniards encountered had no resistance to European diseases such as smallpox and measles. Within half a century the population of Mexico had been decimated--cut from more than 30 million to less than 3 million--as a result of diseases that had been introduced by the Europeans. 

   When a disease causes illness or death to one individual, the social effect is minimal. When huge segments of a population become ill and perhaps die from an outbreak of disease, the social, economic, and political consequences can be enormous. Outbreaks of disease in a significant proportion of a population are called epidemics--from a Greek term meaning "prevalent among the people." When an epidemic afflicts a high proportion of the population over a wide geographic area, it is called a pandemic--literally "all the people." When a disease such as measles or mumps is restricted to a particular region or group of people and, over a long period of time, causes few deaths, then it is considered endemic, or localized. Such infections are also called childhood diseases. Once individuals recover from a childhood disease, they become immune to it. The disease must therefore wait for a new generation to appear or for new groups of children to be born before it becomes active again. 

   A plague is another matter altogether. Plague, in the strictest sense, is a name for a specific and deadly disease called bubonic plague, which is caused by the bacillus Pasteurella pestis. (The bacillus is sometimes called Yersinia pestis, after its discoverer Alexandre Yersin.) The name bubonic is derived from one of the disease's characteristics, the formation of buboes, or inflamed lymph glands. The plague is mainly a disease of rodents. In humans the disease is contracted by contact with the fleas of infected rodents. The disease caused by Pasteurella pestis takes three forms in humans: bubonic; pneumonic, in which the lungs are most seriously infected; and septicemic, in which the bloodstream is so severely infected that the victim dies before the bubonic or pneumonic stages can occur. 

   Plagues and epidemics are studied in the field of medicine called epidemiology. This area of medicine also studies such conditions as fetal alcohol syndrome, which are not diseases in the strict sense. Fetal alcohol syndrome (FAS) refers to the harm done to a fetus when a woman consumes alcohol during pregnancy. This syndrome has become a subject of epidemiology because it has become so widespread. 
 

Epidemics 

   The organism in which a parasite lives is called the host. Some parasites are dependent exclusively upon human hosts for survival and can, therefore, be passed from person to person through some type of contact. Other parasites are transported by means of other agents, such as tiny insects. Malaria, for example, is carried by mosquitoes, and the plague is borne by fleas. In many cases the host is not harmed by the parasite--malaria, for example, does not affect the mosquitoes that carry it. 

   Humans, because of the diversity of their genetic makeup, react to epidemic diseases in different ways. Some people have acquired an immunity for a given disease, others have partial immunity, and still others have no immunity. It is possible to have an immunity for one disease while being susceptible to others. Sometimes an immunity can be temporary, as in the case of influenza. Some diseases, such as measles, confer immunity on those who have had them. A few diseases, including chicken pox and rheumatic fever, can infect an individual early in life and then lie dormant for many years before becoming active again in another way. The virus that causes chicken pox, for example, can lie dormant for decades after the childhood infection has subsided only to cause painful shingles (skin sores that follow the paths of nerves) later in life. 

   In the most precise sense, an epidemic is an unusually high incidence of a disease in a specific geographic area within a limited period of time. Epidemics in this sense are of two types. A contact epidemic results from person-to-person contact. Other epidemics are disseminated--that is, they are carried to humans by a nonhuman agent. The agent can be an insect or it can be a contaminated substance, such as milk or water. A disseminated disease, such as the plague, can quickly become a contact epidemic. 

   Epidemics depend for their existence on population densities. They are aided by person-to-person contact among individuals who have never had a specific disease. A disease spreads especially quickly among similarly susceptible groups of people--children in school or young people in the armed forces. 

   After an epidemic a human host population tends to revert slowly from immunity to a condition of susceptibility to a disease. Immune individuals age and die, while new, nonimmune individuals are born. 

   During the course of human history different diseases that depended on person-to-person contact occurred in somewhat confined areas. Once the disease-carrying organism became localized within the area, the human population would gradually develop a resistance to the disease, and the incidence of epidemics would decrease. If a new disease was introduced from outside the area, however, it could take a terrible toll. 

   In time, as worldwide contacts between civilizations increased, most of the common contact diseases were spread everywhere, and most populations developed resistances to them. Progress in medical research has greatly diminished the likelihood of traditional epidemics. There are, however, occasional outbreaks of disseminated diseases, such as cholera and malaria. Prior to the extensive use of vaccines, these periodic breakdowns in immunity were apparent in the wholesale outbreaks of childhood diseases that occurred on an almost annual basis. Such outbreaks, however, are not necessarily epidemic in nature. 

   In the late 20th century the term epidemic has been loosely used to refer to the widespread occurrence of noncontagious illnesses such as cancer and heart disease. The high incidence of acquired immuno- deficiency syndrome (AIDS) in some geographic areas during the 1980s and 1990s was also called an epidemic. 
 

History of the Plague 

   The disease reached the Crimea in 1346 and found its way to Europe in 1347. The outbreak in Europe was the most disastrous epidemic in European history, resulting in more than 25 million deaths--about one quarter of the continent's population. From then on the plague reappeared irregularly in many European cities until the early 18th century, when it suddenly ceased there. No single explanation has ever been given for the plague's rapid disappearance. 

   From the 18th century through the early 19th century the plague continued to exact a toll in Turkey, the Middle East, Egypt, and Greece. There was an outbreak in Russia in 1878-79. A great pandemic began in China's Yunnan Province about 1855. It did not reach the ports of Canton and Hong Kong until May 1894. It was during the epidemic in Hong Kong that Alexandre Yersin and Shibasaburo Kitasato identified the parasite Yersinia pestis independently of each other. The South China ports were responsible for sending the plague around the world by means of rat-infested ships. The disease was carried to North America, South America, and South Africa--places where it had never been established before. Except for Australia and Antarctica, vast populations of disease-carrying rodents appeared on all the continents. 

   Incidence of the plague has been dramatically reduced in the 20th century. Improved sanitation and treatment with streptomycin, tetracycline, or sulfonamides has significantly reduced the death rate. Readers wanting more information on this subject should consult William H. McNeill's 'Plagues and Peoples', first published in 1976.