Epidemiology is more than headlines of outbreaks of Hantavirus, Lassa fever, and Legionnaire’s disease — it’s the study of the distribution of states of health and of the determinants of deviations from health in human populations.¹ Epidemiologists investigate both infectious and noninfectious diseases — chronic conditions such as cardiovascular disease, cancer, and diabetes; acute events or illnesses; and mental and emotional disorders.

Epidemiologists examine people in groups, or aggregates, and determine the characteristics of well populations to uncover trends in the occurrence of diseases and injuries. They rely on information gathered from census data, vital statistics and reportable disease records, registries, environmental monitoring, public health agencies, and nurses. Nurses frequently furnish data through routine documentation regarding employee health, workers’ compensation, and incident reports that describe on-the-job injuries, needlesticks, patient mishaps, and adverse drug reactions. Although some information may be used by the employing institution only, other data is passed on to the Centers for Disease Control and Prevention, the Food and Drug Administration, and other reporting agencies. By identifying cases in their everyday practices, nurses can help to identify people or populations at high risk; monitor the progression of diseases; specify areas of healthcare need; determine priorities, size, and scope of programs; and evaluate their impact.² Data gleaned from epidemiological studies often forms the basis for best practices.

Historical perspective

The study of epidemiology, from the Greek words epi (upon) and demos (people), dates back to the fifth century BC when Hippocrates and others suggested a relationship between environmental factors and the occurrence of disease. The early 19th century marked the formal beginning of epidemiology when its first investigators focused on infectious diseases.

John Snow conducted one of the first epidemiological investigations. During 1848–1849 and 1853–1854, Snow studied environmental elements associated with cholera outbreaks in England. After visiting the homes of many victims who died, he was able to demonstrate an association between cholera deaths in selected districts and sources of contaminated water. As a result of his findings, Snow was able to encourage improvements in the water supply long before the organism for cholera was isolated.³ Since then, the field has evolved into a discipline of intricate data gathering and analysis that provides the basis for public health practices and policies today.

Basic terminology and concepts

When determining the distribution of any disease or injury, epidemiologists examine incidence, prevalence, and mortality rates. An incidence rate is the rate at which disease occurs in a population at risk over one year.⁴ For example, epidemiologists might estimate that 70 newly diagnosed cases of breast cancer will occur during the next 12 months in Shock City, U.S. Because no nationwide cancer registry exists, a prediction of cancer may be derived from U.S. census estimates of the population for the current year and data from cancer registries covering approximately 26% of the population, known as the Surveillance, Epidemiology, and End Results (SEER) program.⁵ The incidence rate would then be calculated as follows:

In contrast, the prevalence rate, which includes both new and old cases during a specified period of time,⁴ would be higher.

Mortality or death rates refer to the number of deaths for either all causes or for a specific disease, condition, or age group. Other frequently calculated mortality rates, including age-specific death rate, cause-specific death rate, maternal mortality rate, and neonatal and infant mortality rates,⁴ provide information about the health of a population, as well as the country in general. For example, high infant mortality rates (the number of deaths under the age of 1 year divided by the number of live births during a given year) may reflect poor economic conditions, inadequate healthcare, poor nutrition and sanitation, and ineffective parental education. A variety of mathematical tools exist for more advanced epidemiological analysis available to healthcare practitioners on EpiTools .

Natural history of disease

An examination of the natural history of a disease helps to identify approaches to prevention, treatment, and control. The progress of a disease is influenced by the epidemiological triangle, which consists of the disease-producing agent, the affected host, and the environment. Before the disease actually occurs — the prepathogenesis period — the interaction of these three components may alter the host’s susceptibility. For example, the combination of smoking cigarettes and drinking alcohol enhances the likelihood of developing cancer of the mouth.⁶ In contrast, during the pathogenesis period, pathological changes leading to disease take place in the host. In the case of oral cancer, symptoms such as a sore that bleeds easily and does not heal, persistent red or white patches, or difficulty chewing may appear.

Levels of prevention

Armed with information about the cause of disease, epidemiologists can identify populations at risk and employ levels of prevention to develop strategies that prevent, diagnose, and manage the occurrence. Three levels of prevention are used:

Primary prevention minimizes disease or injury either by reducing exposure or by altering susceptibility during the prepathogenesis period before pathological changes occur in a host.¹ Activities are aimed at health promotion and healthy living conditions; specific protection for individuals, such as personal hygiene and sanitation; and safeguards against environmental hazards and accidents. Other examples include scheduled immunizations, provisions for handling toxic waste, and not smoking. Nurses participate at this level as health educators and counselors who promote healthy lifestyles.

Secondary prevention focuses on early detection and prompt treatment of a disease or injury to limit disability and to prevent and/or reduce complications. Screening is the most common form of secondary prevention.¹ Secondary prevention activities occur during the pathogenesis period when the disease is present, detectable, and more amenable to treatment.¹ Screening for breast and cervical cancer, hypertension, lead poisoning, and sexually transmitted diseases are examples. Nurses often contribute to secondary prevention by assessing asymptomatic individuals and identifying early cases.

Tertiary prevention focuses on rehabilitation and limitation of disability once the disease has already occurred. For example, tertiary prevention might encompass rehabilitative measures for a stroke patient, or prescribe arm exercises for a post-mastectomy patient. Nursing interventions could include educating to prevent complications from a disease or injury, providing direct nursing care, and making referrals to resources that aid in reducing or eliminating further complications or disability.

Gathering data

Preventive efforts are only as effective as the information on which they are based. Analysis of current epidemiological situations and future predictions are dependent on accurate data gathered through descriptive, analytical, and experimental studies.

Descriptive studies are observations of when and where diseases or injuries occur and who they affect (person, place, and time). They describe the onset and duration of events and seasonal trends in relationship to physical, economic, and family environments.³ Descriptions frequently include the age, gender, ethnicity, socioeconomic status, and overall health status of the affected individuals, as well as personality traits, environmental exposures, and personal lifestyle factors, such as smoking, drinking, and health history. For example, family history provides one of the most widely recognized risk factors for colon cancer: Numerous studies have noted greater risk of developing colon cancer among individuals whose family members had a diagnosis of colon cancer or polyps and inflammatory bowel disease. This observation has provided a basis for screening guidelines, which recommend that individuals with a family history of colon cancer should begin colorectal cancer screening earlier and/or undergo screening more often.⁷

Descriptive studies furnish information about the occurrence of health and illness that nurses can use to plan health-related programs in both institutional and community-based settings. They establish a basis for making decisions about the creation, allocation, and discontinuation of healthcare resources.

Analytical studies use data from descriptive research to formulate and answer specific questions about variations in incidence and prevalence of diseases and injuries in different populations. The primary purpose is to examine associations and test hypotheses. Differences in purpose, time frame, strengths, and limitations determine if these studies are cross-sectional, case-control, or cohort.³

Cross-sectional, or prevalence studies, examine relationships between disease and other characteristics of a specific population at one point in time. Although a cross-sectional study may demonstrate the simultaneous occurrence of a risk factor and a particular health outcome during the same time period, it doesn’t necessarily establish a cause and effect relationship. For example, even though researchers might find frequent coexistence of acute bronchitis and chronic obstructive pulmonary disease when examining patients’ health histories and X-rays, they can’t be sure which condition came first.

Case-control or retrospective studies investigate disease by using two groups — cases (people who have the condition) and controls (those who don’t) — selected from institutional or community settings. Because this type of study can be inexpensive, uses small samples, and examines multiple exposure factors for a single health condition or disease, it is a good design for examining chronic conditions, rare diseases, and long-term effects related to exposure. Researchers might use a case-control design to examine factors related to toxic shock syndrome; for example, they might compare the use of tampons between case and control groups. The availability of information about past exposures or events can limit the results. Other disadvantages include difficulties in obtaining appropriate control groups and establishing an association between disease exposure and outcomes.³

Cohort studies, also called prospective or longitudinal studies, involve the use of groups of people who are presumed to be free of a disease or condition but differ in the extent of exposure to the studied factor. Subjects are followed over a period of time to evaluate the development of disease. Because cohorts are chosen before the disease develops, researchers are more likely to establish a time sequence between exposure and outcome. An example of a cohort study is the Framingham Study, an investigation studying factors associated with the risk of coronary heart disease.¹ Beginning in 1949, the Framingham Study included a representative sample of 5,209 men and women, ages 30 to 59, residing in Framingham, MA. Participants in the study were examined every other year for evidence of coronary heart disease for more than 30 years. Participants were also examined for diabetes, high blood pressure, abnormal blood cholesterol levels, smoking, and sedentary activity, all deemed to be risk factors for coronary heart disease. This historic cohort study provided much of what is currently known about coronary heart disease and associated risk factors.¹ (See sidebar)

Experimental studies are designed as a result of previous research that indicates a specific factor that may be related to a specific outcome, such as an illness or injury. An experimental study requires the manipulation or control of conditions under which the investigation is conducted. The researcher randomly assigns subjects to either the treatment group or the control group for comparison. Randomization, meaning that every subject has an equal chance of being assigned to either group, takes place to ensure that both the treatment and control groups will be comparable at the onset of the epidemiological study. Randomization helps to reduce selection bias and ensure that when confounding occurs, it is equally distributed between the groups.³ Researchers who conduct experimental studies are responsible for developing and implementing specific guidelines and procedures that are used to ensure the protection of human subjects.

The study of tamoxifen and raloxifene (STAR) is an example of an experimental study. The STAR study is designed to determine whether raloxifene is as effective as tamoxifen in reducing breast cancer risk.⁸ Tamoxifen, an antiestrogen drug, has been successful in preventing breast cancer recurrence and preventing the development of new breast cancer. In the Breast Cancer Prevention Trial (BCPT), tamoxifen was proven to reduce breast cancer incidence by 49% in women with an increased risk of the disease. In a large osteoporosis trial, it was found that those taking raloxifene, which prevents osteoporosis, had a reduced incidence of breast cancer.⁸

Enrollment in the study began July 1, 1999,⁸ By the end of October 2001, 11,307 more women were enrolled. Interested women complete a risk assessment screen to determine eligibility for participation in the study. The participants need to be postmenopausal, at least age 35, have an increased risk for breast cancer as determined by their age, family history of breast cancer, personal medical history, age at first menstrual period, and age at first live birth. The participants are randomly assigned to take one of the drugs daily for five years and neither the participant or her physician know which drug is being taken. Participants have regular check-ups including mammograms and gynecological exams.⁸

Epidemiology has expanded from the study of infectious diseases to include noninfectious diseases, acute and chronic conditions, and injuries. Additional information on available databases and disease entities can be found online at virtual libraries. Because the primary objective of epidemiology — to improve the health and well-being of populations — is congruent with the goals of nursing, nurses are both contributing to this important field and using findings in the form of implementing best practices. Many practitioners are directly involved in data collection or in independent research addressing causative factors of illness and disability. Nurses have found that epidemiological discoveries can improve their clinical decisions, hone the process of identifying populations at risk, and more accurately pinpoint the planning of health services and policies.