Epidemiology is the study of the origin and cause of disease among populations.
Epidemiology is an increasingly important part of public health because rapidly increasing human populations provide a fertile breeding ground for microbes. As the planet becomes more crowded and the distances that separate communities become smaller, infectious diseases that were once limited by geographic isolation now spread globally. In addition, many viruses and bacteria constantly evolve and shift among populations and across species. Epidemiologists use an array of skills to trace the source of disease.
The epidemiologist is concerned with the interactions of organisms and their environments. Environmental factors related to chronic and infectious diseases may include geographical features, climate, the concentration of pathogens in soil and water, or other factors. Some chronic diseases are infectious (e.g., cervical cancer caused by human papilloma virus [HPV], liver cancer caused by hepatitis B or C virus, and ulcers caused by H. pylori infections). Epidemiologists determine the numbers of individuals affected by a disease, the environmental contributions to disease, the causative agent or agents of disease, and the transmission patterns and lethality of disease.
Epidemiology is commonly thought to be limited to the study of infectious diseases, but that is only one aspect of the medical specialty. The epidemiology of the environment and lifestyles has been studied since Hippocrates' time. The first Nobel Prize in Medicine awarded to an epidemiologist was to Richard Doll for his work linking smoking to cancer. More recently, scientists have broadened the worldwide scope of epidemiology to studies of violence, of heart disease due to lifestyle choices, and to the spread of disease because of environmental degradation.
Epidemiologists at the Epidemic Intelligence Service (EIS) of the Centers for Disease Control and Prevention (CDC) have played important roles in landmark epidemiological investigations. Those include an investigation of Legionnaires disease in 1976, identification of tampons as a risk factor for toxic-shock syndrome, and investigation of the first cluster of cases that came to be called acquired immunodeficiency syndrome (AIDS). EIS officers are increasingly involved in the investigation of non-infectious disease problems, including the risk of injury associated with all-terrain vehicles and cluster deaths related to flour contaminated with parathion.
In the 1980s, epidemiologists connected AIDS to sexual contact, then to contaminated blood supplies, then proved the AIDS virus could cross the placental barrier, infecting babies born to HIV-infected mothers. Since the identification of the HIV virus responsible for AIDS, multiple genetic studies have established that the HIV virus passed into humans in Africa from chimpanzees that were probably butchered for meat about 100 years ago. Before being identified in the United States in the early 1980s, what eventually became known as AIDS syndrome was effectively hidden because it existed in people living in remote areas and occurred at low population levels. What is now known as AIDS was previously misdiagnosed as other diseases. For example, doctors in Africa often listed leukemia as the cause of death for persons who had AIDS.
The epidemiological classification of disease deals with the incidence, distribution, and control of disorders of a population. Using the example of typhoid, a disease spread through contaminated food and water, scientists first must establish that the disease observed is truly caused by Salmonella typhosa, the typhoid organism. Investigators then must determine the number of cases, whether the cases were scattered over the course of a year or occurred within a short period, and the geographic distribution. It is critical that the precise locations of the diseased patients be established. In a hypothetical case, two widely separated locations within a city might be found to have clusters of cases of typhoid arising simultaneously. It might be that each of these clusters revolved around a family unit, suggesting that personal relationships could be important. Further investigation might disclose that all of the infected persons had dined at one time or at short intervals in a specific home, and that the person who had prepared the meal had visited a rural area, suffered a mild attack of the disease, and now was spreading it to family and friends by unknowingly contaminating of food.
An epidemic of cholera in the West African nation of Guinea-Bissau was tracked by CDC researchers using maps, interviews, and old-fashioned footwork through the country. Aninvestigator eventually tracked the source of the cholera outbreak to contaminated shellfish. Epidemiologists also use high-tech tools. During a 2010 Salmonella outbreak in the United States, epidemiologists used pulse field gel electrophoresis (PFGE) to trace the source of the outbreak, which was linked to eggs sold to consumers under twenty-five different brand names by egg-producing farms that sold eggs to multiple packagers.
Changes in environment or social infrastructure can also fuel epidemics. For example, a breakdown in sanitation frequently offers conditions favorable to the spread of disease. An epidemic can result from an alteration in the environment that normally acts to limit disease.
In other cases, ecological imbalances can also increase the spread of disease. For example, an increase in mosquitoes, vectors for a number of diseases, can offer continuing challenges to public health officials. In addition to tracking the origins of disease, epidemiologists may investigate the factors causing the increase in vector populations.
Lyme disease, which was tracked by epidemiologists from man to deer to the ticks that infest deer, is directly related to environmental changes. The lyme disease spirochete probably has been infecting ticks for a long time; museum specimens of ticks collected on Long Island in the l940s were found to be infected. Since then, tick populations in the Northeast have increased dramatically, triggering the epidemic. Epidemiologists and biologists concluded that the increase in ticks was linked to reforestation practices. Deer populations in those areas exploded, as did the numbers of Ixodes dammini ticks that feed on deer. The deer do not become ill, but when a tick bite infects a human host, the result can be lyme disease, which in severe cases causes crippling arthritis and memory loss.
Epidemiologists also study factors causing heart disease and cancer, developing a deeper understanding of the environmental, nutritional, and lifestyle factors that contribute to disease.
See also Centers for Disease Control and Prevention ; Cholera ; HIV/AIDS ; Malaria ; Pandemic ; Sanitation ; Viruses ; World Health Organization .
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Alyson C. Heimer, MA