During the 14th century about 25 million people in Europe died from a disease known as the plague. Since that time the term plague has sometimes been applied to any outbreak of a disease that strikes a large proportion of a population. Such an outbreak is also called an epidemic. Outbreaks of plague do not happen often nowadays. Scientists, however, track the number of cases of plague and other diseases in an effort to prevent possible epidemics.

Plague

Plague is caused by the bacterium Yersinia pestis (also known as Pasteurella pestis). It is mainly a disease of rodents, especially rats. The fleas that spread the disease from an infected rodent to another rodent can also give it to a person they bite. Infected people sometimes spread the disease to others through droplets released when they cough. Rats traveling aboard ships have spread plague to all areas of the world.

The most common form of plague in humans is the bubonic plague. Swelling of the lymph nodes is often the first sign of the infection. Other symptoms include shivering, vomiting, headache, pain in the back and limbs, and a high fever. Other forms of plague affect the lungs and the bloodstream.

Certain antibiotic medicines are used to help people with plague symptoms. A vaccine is available to people who live in areas where they are likely to come into contact with rodents and their fleas. Governments and health groups also try to keep the numbers of rats and fleas from getting too large.

Epidemics

A widespread outbreak of the plague like that of 14th-century Europe is just one example of an epidemic. Epidemic diseases are caused by tiny organisms called parasites that live on or in another organism. Parasites include viruses as well as certain bacteria, fungi, protozoa, and worms. Parasites tend to thrive in the warmer climates of places such as Africa, India, and Latin America.

The organism in which a parasite lives is called the host. Some parasites make humans their only host. These parasites can be passed from person to person through some type of contact. Other parasites are passed to humans through something else, such as tiny insects.

The diseases that cause epidemics are contagious, meaning that they are spread easily. People sometimes acquire immunity to a disease, meaning they will not catch it. When a new disease is introduced into a region, however, it is unlikely that much of the population will have immunity.

As people throughout history have moved into new areas, they have brought along the diseases of their homelands. Natives of these conquered lands often have caught these new diseases, such as smallpox and measles. The effects have been huge. For example, 16th-century Mexico lost about 90 percent of its population to diseases brought in by Europeans.

In time, as worldwide contacts between civilizations increased, most of the common contact diseases were spread everywhere. Most populations have developed immunity to them. Improved sanitation and medicine have also helped limit epidemics.

Epidemics still sometimes happen, however. Cholera and malaria are two diseases that become widespread from time to time. An influenza epidemic that began in 1918, at the end of World War I, killed about 20 million—more than double the number of people who died in the war itself. The high number of cases of AIDS in some areas since the 1980s has led to this disease being called an epidemic. New diseases can still develop and spread rapidly as well. In 2003 a flulike illness known as severe acute respiratory syndrome (SARS) quickly became an epidemic. It spread from Asia, where it began, to many other parts of the world within a few months. Because of improved health-care practices, however, it was also quickly contained. In the 20th and 21st centuries worries arose that terrorists might try to introduce diseases into a population in the hope of causing an epidemic.