Definition: Communicable human diseases caused by pathogenic microorganisms
Significance: During North America’s colonial era, immigrants from Europe and Africa imported many contagious diseases that wreaked havoc on not only Native American populations but also nonimmunized colonists. Successive waves of disease-carrying immigrants during the nineteenth century set off epidemics ranging from cholera to plague, despite ever more effective public health measures, and encountered effective anti-immigrant sentiment and action. During the early twenty-first century, visitors as well as immigrants posed threats to U.S. public health as carriers of new diseases and new strains of old diseases.
Every person and every community lives in an environment filled with bacteria, viruses, fungi, and parasites, many of which carry pathogens potentially lethal to humans. People who live for many years in the same area and with the same neighbors develop effective immune system defenses against the commonly occurring pathogens. Sometimes they pass their immunity along to subsequent generations genetically. When a new pathogen is inserted into a community by changes in the environment or the intrusion of new people, the effects may be devastating, as existing members of the community may have limited or no developed biological defenses. Unlike noninfectious diseases such as diabetes or cancer, an infectious disease can be passed among members of a community by the actions of carriers of pathogens. These carriers might include tainted foods or water; insects, parasites, and their droppings; and infected people. During the centuries before germ theory made modern medicine an effective counter to most infectious diseases, there was little understanding of pathogens and carriers, and little that any human community could do to defend against them.
Joseph P. Byrne
See also: Acquired immunodeficiency syndrome; African immigrants; Ellis Island; Eugenics movement; Globalization; Great Irish Famine; Haitian boat people; Health care; Smuggling of immigrants; Sweatshops; Transportation of immigrants; “Undesirable aliens”; World migration patterns.