The Epidemiologic Triangle

However, the importance of what an epidemiologist does can be seen in their national unemployment rate. According to the Bureau of Labor and Statistics, the national unemployment rate was 5. The national unemployment rate of health services professionals, as a whole, was 4 percent.

The Epidemiologic Triangle

However, the national unemployment rate of epidemiologist was only 1. Even though the overall job growth prospects for epidemiologists as a whole is anticipated to grow 10 percent between and , one of the primary factors in that estimate is the assumption that states and municipalities will continue to keep government spending at a minimum. However, epidemiologists can often find work in related fields. The below related careers show higher growth rates and increased potential for those who desire a career in epidemiology or a similar field:.

Survey researchers design surveys and analyze data from those surveys. Topics of surveys can range from public opinion and education to medical and scientific surveys. Survey researchers must also be able to effectively convey their findings. A statistician collects, studies, analyzes and presents data, usually to help understand a concept or dilemma.

What Is the Epidemiologic Triangle?

Areas of data collection can include anything, including medical research, politics and business. Those in community health work with the general public in order to teach healthy behaviors and implement public health strategies. Duties often include collection and analysis of health related data as well as implementation of community wellness programs. The growth potential of epidemiologists depends not only on budgetary concerns of state and local governments and the area of specialization, but also geographical location of the position. Certain regions of the United States are expected to grow faster, or slower, than the projected 10 percent from to When deciding on a career in epidemiology, there are questions you can ask yourself to decide which specific epidemiology job is right for you.

Factors to consider include your likes, dislikes, strengths, weaknesses and personality traits. Epidemiology is a very important career. However, there is the general idea that epidemiologists are statisticians who work in a government office crunching numbers to find patterns in health related problems. Like many other professions, what an epidemiologist does is actually much more varied than what someone might think.

Although analyzing statistics and research data is a large part of the day-to-day work, the actual job duties can vary greatly. The below graphic illustrates other interesting careers for those intrigued by epidemiology. Veterinary epidemiologists study the cause and spread of diseases within groups of animals other than humans.

Additionally, private enterprises such as those involved in the meat production industry rely on the expertise of veterinary epidemiologists to prevent diseases from harming the animals and monitoring their overall health. Typical job duties of a veterinary epidemiologist include studying how diseases are transmitted among animals and between animals and humans, evaluating the efficacy of animal vaccines and medicines and analyzing information obtained through research and observation in order to draw conclusions about a given illness or disease.

Due to time-intensive nature of data gathering, much of the information analyzed by veterinary epidemiologists is obtained from veterinarians or veterinarian technicians, rather than directly from the field. Most veterinary epidemiologists are also veterinarians. Therefore, one of the veterinary epidemiologist requirements is to obtain a veterinary medicine degree.

Next, the individual works to become licensed to practice medicine and then obtain additional training and education to become an epidemiologist. Finally, the veterinary epidemiologist can become board certified. While becoming board certified is not mandatory in order to become a veterinary epidemiologist, it is strongly recommended. A veterinary epidemiologist becomes board certified by becoming a diplomat in the American College of Veterinary Preventive Medicine, achieving two years of experience in the field of epidemiology, seeing articles published in scientific journals and presenting three professional references.

Finally, the veterinary epidemiologist must pass the board certification exam. In order to become a successful epidemiologist, there are important tools, skills and personality traits one should possess. For example, a strong grasp of math theories and statistical concepts is a must, even if one does not actively conduct research and analyze data. Below is a list of skills and traits that could be useful in launching a successful epidemiology career.

Perhaps the single most important skill an epidemiologist must have is the ability to use and understand statistics. Gathering data is one thing, but being able to create credible, real world conclusions from the data is much more important and useful. Also, even if a particular epidemiologist is not involved in data creation or analysis, she or he must still be able to critically analyze the data and test its significance, accuracy or both.

Equally as important as proficiency of statistical concepts, an understanding of medical and biological processes is critical for effective analysis of how to prevent and treat diseases. Finding a treatment or vaccine for a disease is not possible without first understanding how the disease works in the human body. Even if an epidemiologist is able to use and understand statistical tools or biological concepts, they must also be able to know when to use such tools and identify issues in a given public health problem. Epidemiologists are paid to find patterns, draw conclusions and develop plans.

These ideas must be adequately conveyed to others. Finding a means to prevent a public health problem is meaningless unless the epidemiologist is able to communicate that method of prevention to the proper officials or general public. Epidemiologists must be accurate with their data gathering and analysis, as well as able to identify nuances in numerical data. They must also be able to identify public health issues that may not be readily observable to identifiable to the average medical professional.

Much of the data collection, storage and analysis is done by computer and databases. An epidemiologist must know how to use the technology to access and process this information. A familiarity of information technology advancements such as Big Data, data mining and predictive analytics can also be very useful.

So how do these skills relate to a given salary for an epidemiologist? According to Payscale. Note that two of the skills, research analysis and data analysis, are highly contingent on critical thinking skills and an understanding of statistical, medical and biological concepts. It is no surprise that these are the skills that take extra education and involve the more difficult classes in post-secondary education.

Epidemiologists are well-educated and very skilled individuals who must undergo extensive training in order study diseases and how they affect a given group of people. For example, many epidemiologists have masters or doctorate level degrees as well as years of practical experience.

However, salary. Type of industry is a very important factor that affects what an epidemiologist salary will be. Even though state and local government epidemiologists make less than the median, there are far more jobs in those areas than in the non-governmental sector. The below chart summarizes the differences based on industry:. According to salary. The below chart containing data from the Bureau of Labor and Statistics summarizes the differences based on state of employment:. Unlike seasoned epidemiologists, fresh graduates lack the years of experience and a substantial professional network to find employment.

Luckily, there are several ways a newly minted epidemiologist can engage in a successful job search.

Principles of Epidemiology: Lesson 5 Quiz|Self-Study Course SS|CDC

One of the best ways to research the job market and find the right epidemiology job is to consult with the career services office. They will have valuable contacts of those already in epidemiology or related fields that can help get your foot in the door with a resume or interview. The career services office will also be able to provide advice on effective job search strategies.

Another effective strategy is to speak with classmates and professors who have been or are seeking to be in the epidemiology field. Classmates can provide valuable information as to successes and failures of the epidemiologist job search process. They may also know who is currently hiring and help narrow down where to send job applications. Professors can also be extremely valuable, serving as references for job applications or networking contacts to get a job interview or an informal job recommendation. Joining epidemiological organizations can provide valuable opportunities to both network and gather additional experience.

These organizations will give valuable exposure to what epidemiologists do as well as provide opportunities for experience. Some even provide volunteer opportunities, such as the Association for Professionals in Infection Control. Finally, there are training and fellowship opportunities. For example, the Center of Disease Control and Prevention has the following four programs for epidemiology students and graduates:. This could include a policy position or a position at a regulatory agency, even if it is for only a limited amount of time.

Understanding this perspective will be invaluable to you no matter what you choose to pursue longer term. Content Navigation. Select a Category -- Select a Category Select a Subject -- Select a Subject What Is Epidemiology? Inside The Lab: Epidemiology Resources: Sponsored School. Featured Programs: Featured Programs. Development and improvement of medical research methodology.

Survey Researcher Survey researchers design surveys and analyze data from those surveys. Statistician A statistician collects, studies, analyzes and presents data, usually to help understand a concept or dilemma. Community Health Worker Those in community health work with the general public in order to teach healthy behaviors and implement public health strategies. The most commonly studied rate is the attack rate: Epidemiology can accurately describe a disease and many factors concerning its occurrence before its cause is identified.

For example, Snow described many aspects of the epidemiology of cholera in the late s, fully 30 years before Koch described the bacillus and Semmelweis described puerperal fever in detail in and recommended appropriate control and prevention measures a number of years before the streptococcal agent was fully described.


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One goal of epidemiologic studies is to define the parameters of a disease, including risk factors, in order to develop the most effective measures for control. This chapter includes a discussion of the chain of infection, the three main epidemiologic methods, and how to investigate an epidemic Table Proper interpretation of disease-specific epidemiologic data requires information concerning past as well as present occurrence of the disease. An increase in the number of reported cases of a disease that is normal and expected, representing a seasonal pattern of change in host susceptibility, does not constitute an epidemic.

Therefore, the regular collection, collation, analysis, and reporting of data concerning the occurrence of a disease is important to properly interpret short-term changes in occurrence. A sensitive and specific surveillance program is important for the proper interpretation of disease occurrence data. Almost every country has a national disease surveillance program that regularly collects data on selected diseases.

The quality of these programs varies, but, generally, useful data are collected that are important in developing control and prevention measures. There is an international agreement that the occurrence of three diseases—cholera, plague, and yellow fever—will be reported to the World Health Organization in Geneva, Switzerland. Public Health Service, and the state health officers of all 50 states have agreed to report the occurrence of 51 diseases weekly and of another 10 diseases annually from the states to the CDC.

Many states have regulations or laws that mandate reporting of these diseases and often of other diseases of specific interest to the state health department. The methods of case reporting vary within each state. Passive reporting is one of the main methods. In such a case, physicians or personnel in clinics or hospitals report occurrences of relevant diseases by telephone, postcard, or a reporting form, usually at weekly intervals. In some instances, the report may be initiated by the public health or clinical laboratory where the etiologic agent is identified.

Some diseases, such as human rabies, must be reported by telephone as soon as diagnosed. In an active surveillance program, the health authority regularly initiates the request for reporting. The local health department may call all or some health care providers at regular intervals to inquire about the occurrence of a disease or diseases. The active system may be used during an epidemic or if accurate data concerning all cases of a disease are desired. The health care provider usually makes the initial passive report to a local authority, such as a city or county health department.

The four basic study designs

This unit collates its data and sends a report to the next highest health department level, usually the state health department. The number of cases of each reportable disease are presented weekly, via computer linkage, by the state health department to the CDC. Data are analyzed at each level to develop needed information to assist public health authorities in disease control and prevention. For some diseases, such as hepatitis, the CDC requests preparation of a separate case reporting form containing more specific details.

In addition, the CDC prepares and distributes routine reports summarizing and interpreting the analyses and providing information on epidemics and other appropriate public health matters. Most states and some county health departments also prepare and distribute their own surveillance reports. The CDC also prepares more detailed surveillance reports for specific diseases, as well as an annual summary report, all of which can also be obtained through the Massachusetts Medical Society.

Infection is the replication of organisms in the tissue of a host; when defined in terms of infection, disease is overt clinical manifestation. In an inapparent subclinical infection, an immune response can occur without overt clinical disease. A carrier colonized individual is a person in whom organisms are present and may be multiplying, but who shows no clinical response to their presence. The carrier state may be permanent, with the organism always present; intermittent, with the organism present for various periods; or temporary, with carriage for only a brief period.

Dissemination is the movement of an infectious agent from a source directly into the environment; when infection results from dissemination, the source, if an individual, is referred to as a dangerous disseminator. Infectiousness is the transmission of organisms from a source, or reservoir see below , to a susceptible individual. A human may be infective during the preclinical, clinical, postclinical, or recovery phase of an illness.

The incubation period is the interval in the preclinical period between the time at which the causative agent first infects the host and the onset of clinical symptoms; during this time the agent is replicating. Transmission is most likely during the incubation period for some diseases such as measles; in other diseases such as shigellosis, transmission occurs during the clinical period. The spectrum of occurrence of disease in a defined population includes sporadic occasional occurrence ; endemic regular, continuing occurrence ; epidemic significantly increased occurrence ; and pandemic epidemic occurrence in multiple countries.

The chain of infection includes the three factors that lead to infection: These links should be characterized before control and prevention measures are proposed. Environmental factors that may influence disease occurrence must be evaluated. Summary of important aspects involved in the chain of any infection. The etiologic agent may be any microorganism that can cause infection.

The pathogenicity of an agent is its ability to cause disease; pathogenicity is further characterized by describing the organism's virulence and invasiveness. Virulence refers to the severity of infection, which can be expressed by describing the morbidity incidence of disease and mortality death rate of the infection.

An example of a highly virulent organism is Yersinia pestis, the agent of plague, which almost always causes severe disease in the susceptible host. The invasiveness of an organism refers to its ability to invade tissue. Vibrio cholerae organisms are noninvasive, causing symptoms by releasing into the intestinal canal an exotoxin that acts on the tissues. In contrast, Shigella organisms in the intestinal canal are invasive and migrate into the tissue.

No microorganism is assuredly avirulent. An organism may have very low virulence, but if the host is highly susceptible, as when therapeutically immunosuppressed, infection with that organism may cause disease. For example, the poliomyelitis virus used in oral polio vaccine is highly attenuated and thus has low virulence, but in some highly susceptible individuals it may cause paralytic disease. Other factors should be considered in describing the agent. The infecting dose the number of organisms necessary to cause disease varies according to the organism, method of transmission, site of entrance of the organism into the host, host defenses, and host species.

Another agent factor is specificity; some agents for example, Salmonella typhimurium can infect a broad range of hosts; others have a narrow range of hosts. Styphi, for example, infects only humans. Other agent factors include antigenic composition, which can vary within a species as in influenza virus or Streptococcus species ; antibiotic sensitivity; resistance transfer plasmids see Ch. The reservoir of an organism is the site where it resides, metabolizes, and multiplies. The source of the organism is the site from which it is transmitted to a susceptible host, either directly or indirectly through an intermediary object.

The reservoir and source can be different; for example, the reservoir for S typhi could be the gallbladder of an infected individual, but the source for transmission might be food contaminated by the carrier. The reservoir and source can also be the same, as in an individual who is a permanent nasal carrier of S aureus and who disseminates organisms from this site. The distinction can be important when considering where to apply control measures. The method of transmission is the means by which the agent goes from the source to the host.

The four major methods of transmission are by contact, by common vehicle, by air or via a vector. In contact transmission the agent is spread directly, indirectly, or by airborne droplets. Direct contact transmission takes place when organisms are transmitted directly from the source to the susceptible host without involving an intermediate object; this is also referred to as person-to-person transmission. An example is the transmission of hepatitis A virus from one individual to another by hand contact.

Indirect transmission occurs when the organisms are transmitted from a source, either animate or inanimate, to a host by means of an inanimate object. An example is transmission of Pseudomonas organisms from one individual to another by means of a shaving brush. Droplet spread refers to organisms that travel through the air very short distances, that is, less than 3 feet from a source to a host. Therefore, the organisms are not airborne in the true sense.

An example of a disease that may be spread by droplets is measles. Common-vehicle transmission refers to agents transmitted by a common inanimate vehicle, with multiple cases resulting from such exposure. This category includes diseases in which food or water as well as drugs and parenteral fluids are the vehicles of infection.


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Examples include food-borne salmonellosis, waterborne shigellosis, and bacteremia resulting from use of intravenous fluids contaminated with a gram-negative organism. The third method of transmission, airborne transmission, refers to infection spread by droplet nuclei or dust. To be truly airborne, the particles should travel more than 3 feet through the air from the source to the host. Droplet nuclei are the residue from the evaporation of fluid from droplets, are light enough to be transmitted more than 3 feet from the source, and may remain airborne for prolonged periods.

Tuberculosis is primarily an airborne disease; the source may be a coughing patient who creates aerosols of droplet nuclei that contain tubercle bacilli. Infectious agents may be contained in dust particles, which may become resuspended and transmitted to hosts. An example occurred in an outbreak of salmonellosis in a newborn nursery in which Salmonella -contaminated dust in a vacuum cleaner bag was resuspended when the equipment was used repeatedly, resulting in infections among the newborns.


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The fourth method of transmission is vector borne transmission, in which arthropods are the vectors. Vector transmission may be external or internal. External, or mechanical, transmission occurs when organisms are carried mechanically on the vector for example, Salmonella organisms that contaminate the legs of flies. Internal transmission occurs when the organisms are carried within the vector. If the pathogen is not changed by its carriage within the vector, the carriage is called harborage as when a flea ingests plague bacilli from an infected individual or animal and contaminates a susceptible host when it feeds again; the organism is not changed while in the flea.

The other form of internal transmission is called biologic. In this form, the organism is changed biologically during its passage through the vector for example, malaria parasites in the mosquito vector. An infectious agent may be transmitted by more than one route. For example, Salmonella may be transmitted by a common vehicle food or by contact spread human carrier. Francisella tularensis may be transmitted by any of the four routes.

The third link in the chain of infection is the host. The organism may enter the host through the skin, mucous membranes, lungs, gastrointestinal tract, or genitourinary tract, and it may enter fetuses through the placenta. The resulting disease often reflects the point of entrance, but not always: Development of disease in a host reflects agent characteristics see above and is influenced by host defense mechanisms, which may be nonspecific or specific.

Nonspecific defense mechanisms include the skin, mucous membranes, secretions, excretions, enzymes, the inflammatory response, genetic factors, hormones, nutrition, behavioral patterns, and the presence of other diseases. Specific defense mechanisms or immunity may be natural, resulting from exposure to the infectious agent, or artificial, resulting from active or passive immunization see Ch. The environment can affect any link in the chain of infection.

Temperature can assist or inhibit multiplication of organisms at their reservoir; air velocity can assist the airborne movement of droplet nuclei; low humidity can damage mucous membranes; and ultraviolet radiation can kill the microorganisms. In any investigation of disease, it is important to evaluate the effect of environmental factors.

At times, environmental control measures are instituted more on emotional grounds than on the basis of epidemiologic fact. It should be apparent that the occurrence of disease results from the interaction of many factors Table Some of these factors are outlined here. The three major epidemiologic techniques are descriptive, analytic, and experimental.

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Although all three can be used in investigating the occurrence of disease, the method used most is descriptive epidemiology. Once the basic epidemiology of a disease has been described, specific analytic methods can be used to study the disease further, and a specific experimental approach can be developed to test a hypothesis. In descriptive epidemiology, data that describe the occurrence of the disease are collected by various methods from all relevant sources.

The data are then collated by time, place, and person. Four time trends are considered in describing the epidemiologic data. The secular trend describes the occurrence of disease over a prolonged period, usually years; it is influenced by the degree of immunity in the population and possibly nonspecific measures such as improved socioeconomic and nutritional levels among the population.

For example, the secular trend of tetanus in the United States since shows a gradual and steady decline. The second time trend is the periodic trend. A temporary modification in the overall secular trend, the periodic trend may indicate a change in the antigenic characteristics of the disease agent. For example, the change in antigenic structure of the prevalent influenza A virus every 2 to 3 years results in periodic increases in the occurrence of clinical influenza caused by lack of natural immunity among the population.

Additionally, a lowering of the overall immunity of a population or a segment thereof known as herd immunity can result in an increase in the occurrence of the disease. This can be seen with some immunizable diseases when periodic decreases occur in the level of immunization in a defined population. This may then result in an increase in the number of cases, with a subsequent rise in the overall level of herd immunity.

The number of new cases then decreases until the herd's immunity is low enough to allow transmission to occur again and new cases then appear. The third time trend is the seasonal trend. This trend reflects seasonal changes in disease occurrence following changes in environmental conditions that enhance the ability of the agent to replicate or be transmitted. For example, food-borne disease outbreaks occur more frequently in the summer, when temperatures favor multiplication of bacteria.

This trend becomes evident when the occurrence of salmonellosis is examined on a monthly basis Fig. An example of a disease showing a seasonal trend. Reported human Salmonella isolations, by 4-week average, in the United States from to The fourth time trend is the epidemic occurrence of disease. An epidemic is a sudden increase in occurrence due to prevalent factors that support transmission.

A description of epidemiologic data by place must consider three different sites: Therefore, in an outbreak of food poisoning, the host may become clinically ill at home from food eaten in a restaurant. The vehicle may have been undercooked chicken, which became infected on a poultry farm. These differences are important to consider in attempting to prevent additional cases. The third focus of descriptive epidemiology is the infected person.