A disease is described as endemic to the particular geographical area in which humans are known to contract the disease. Many diseases only affect persons who visit particular areas, such as malaria, melioidosis, Rocky Mountain Spotted Fever, African trypanosomiasis and leishmaniasis. People who have never visited areas where a disease is not endemic have a virtually zero chance of having such diseases even if their symptoms otherwise seem to fit. As such, a medical history is not complete unless the patient's entire travel history is included.
Other diseases are not endemic to any particular area. These diseases tend to be more contagious and are easily passed from person to person. Endemic diseases usually require some sort of vector to pass them along, although environmental factors can also play a factor. Influenza and measles, for example, are not endemic to any particular area.
Some diseases that were once widespread and contagious often become endemic and limited when steps are taken to prevent disease transmission. For example, cholera used to be widespread, but now only exists in a few places and rarely spreads. Plague used to spread rapidly, but is now limited to just a few places.
However, the reverse is also true. Diseases that were once endemic to Europe started to spread worldwide in the 15th century with European exploration. For example, smallpox was unknown in the Americas until Europeans arrived in the 16th and 17th century, even though smallpox would have been around when humans first went to the Americas. Conversely, by 1973, the disease was only endemic to Bangladesh and was unknown elsewhere due to vaccination efforts.