Epidemiology is the study of factors that influence the health and illness of populations. The three levels of causation of health problems include individual behavioral level, individual biological level, and political-economic ecological level.

Aims
Epidemiologists aim to understand the causes of health problems by looking at the relationship between agents, hosts, and environmental factors that affects health. Using these information, they also design public health interventions to solve various health problems among populations. They regularly evaluate the health of populations, tries to identify which populations have greater risk from specific causes of diseases, and evaluates the effectiveness of the intervention programs or methods that they create. In essence, epidemiology provides the foundation of public health and preventative medicine practices.

Descriptive/analytic epidemiology
There are two types of epidemiology: descriptive and analytic. Descriptive epidemiology aims to describe the distribution of people who gets sick (versus those who don't) with regards to time, place, and person (TPP). TPP can also be thought of as answering the questions of when, who, and what respectively; in other words it investigates when a health issue or disease first emerged in a population, who in the population are getting sick, and where the disease/health condition seems to have first originated and spread from. Analytic epidemiology then uses the data provided by TPP to make and test hypothesis which aim to determine the cause of an outbreak or disease cluster among a population. Epidemiologist evaluate morbidity (people who are sick or injured) and mortality (people who are killed) to understand the burden of disease among populations.

Surveillance studies
A key aspect of epidemiology is surveillance studies, which provides the numerical data and statistics that epidemiologists use. Surveillance studies are done systematically to monitor the health of populations and helps with identifying any new health problems or disease that may develop, as well as evaluating the effectiveness of existing health intervention measures in place. These studies also rely on TPP to collect data and presents it in simple graphs and tables which are easy to summarize and understand. These studies are especially important in helping guide policy making decisions and helping epidemiologists understand which interventions work and which don't. It is also important for donors as they rely on surveillance data to analyze the usefulness of their investments and whether it is being used effectively or not.

Definition and origin of term

Epidemiology means "the study of what is upon the people". The word derived from the Greek terms epi = upon, among; demos = people, district; logos = study, word, discourse. It applies only to human populations. But the term is used in studies of zoological populations 'epizoology', and plant populations.

History 

Hippocrates was the first who has looked at the relationships between disease and environmental influences. He drew the distinction between 'epidemic' and 'endemic': diseases that are 'visited upon' a population (epidemic) as contrasted with those that 'live within' a population (endemic).

11th century
The Persian physician Avicenna in the 1020s, discovered the contagious nature of tuberculosis and sexually transmitted disease. He noted the distribution of disease through water and soil. Avicenna said that bodily secretion is contaminated by foul foreign earthly bodies before being infected. He introduced the method of quarantine to limit the spread of contagious disease.

Black death
The Black Death (bubonic plague) reached Al Andalus in the 14th century. Ibn Khatima thought infectious diseases were caused by "minute bodies" which enter the human body and cause disease. Another Andalusian-Arabian physician, Ibn al-Khatib (1313–1374) in his treatise On the Plague stated how infectious disease can be transmitted through bodily contact and "through garments, vessels and earrings". Girolamo Fracastoro from Verona suggested these very small, unseeable, particles that cause disease were alive. They were able to spread by air, and multiply. They could be destroyed by fire. He refuted Galen's miasma theory (poison gas in sick people). In 1543, Fracastoro's book De contagione et contagiosis morbis suggested personal and environmental hygiene to prevent disease. The development of a sufficiently powerful microscope by Anton van Leeuwenhoek in 1675 provided visual evidence of living particles consistent with a germ theory of disease.

Great Plague
In 1662 John Graunt analysed the mortality rolls in London before the Great Plague. This gave statistical evidence for and against various theories of disease. Dr. John Snow investigated the causes of the 19th Century Cholera epidemics. He noticed the significantly higher death rates in two areas supplied by Southwark Water Company. He showed the Broad Street pump was the origin of the Soho epidemic, a classic example of epidemiology He used chlorine in an attempt to clean the water and had the pump handle removed. This stopped the outbreak. It was a major event in the history of public health, and the founding event of the science of epidemiology.

19th century
The term 'epidemiology' was first used in 1802 by the Spanish physician Villalba. The term is used now for the description and causation of epidemic diseases, and of disease in general. It can be used for many non-disease health-related conditions, such as high blood pressure and obesity.

In 1847 Hungarian physician Ignaz Semmelweis brought down infant mortality at a Vienna hospital by disinfection. Unfortunately, disinfection did not become widely practiced until British surgeon Joseph Lister 'discovered' antiseptics in 1865 after Louis Pasteur's work. In the early 20th century, mathematical methods were introduced into epidemiology by Ronald Ross and others. In 1954 came the results of a study led by Richard Doll. This gave very strong statistical support to the suspicion that tobacco smoking was linked to lung cancer.

Important concepts/terms 
There are several very key terms that epidemiologists use when discussing population health and disease outbreaks. The following, although not a comprehensive list, provides some of the key concepts that are important to understand when discussing epidemiology.
 Cases: refers specifically to those individual who are sick with a disease/health condition or injured
 Epidemic / Outbreak: is the occurrence of a disease among a population that is in excess (higher rate) than what is expected for that given time and place
  Endemic: a disease or health condition that is present in the population at all times during the year
 Pandemic: a disease that spreads across various regions; also refers to global outbreaks that spreads over multiple continents
 Cluster: refers to group of cases in a specific time and place that's more than what's expected
 Population at risk: refers to those within a population who are particularly susceptible to a certain disease or health condition

It is important to note that an endemic disease or cluster can become an epidemic. An example of this would be with malaria; although malaria is endemic to certain regions in South America, Africa, and South Asia, during certain years or times it can become an epidemic with higher number of cases then usual present in the population. In addition, it is also possible for epidemic or outbreak to progress and become a full-fledged pandemic.

Calculating Disease Rates  
Rates refers to the number of cases occurring during a specific period of time and depends on the population size at that time. Calculating disease rates helps epidemiologists to compare health issues among different populations. The general calculation for determining disease rate is to divide the number of cases or health condition by the number of population at risk during a specific period of time, and multiplying that by 100. However, disease rates can also be differentiated into two different types: prevalence rate and incidence rate.

Prevalence rate refers to the number of both old and new cases in a population during a specific time period, which is divided by the total number of cases in the population. Prevalence rates are useful when dealing with investigations relating to chronic diseases, which last for more than 3 months. On the other hand, incidence rate refers to the number of new health related conditions or cases which is divided by the population at risk. Incidence rates are important in studies involving acute diseases, where symptoms of a disease peak and subside within days or weeks and generally lasts less than 3 months.

Types of Epidemiological Studies 

Epidemiological studies makes use of both experimental and observational studies.

Experimental studies
Experimental studies are ones where the epidemiologist can control and manipulate different variables throughout the experiment. It usually involves a placebo treatment/group. This type of study is used when epidemiologists are trying to determine the cause of a health issue/disease or evaluating the effectiveness of a cure or interventions.

Observational studies
Observational studies include descriptive and analytical studies; descriptive studies investigates epidemiological cases with regards to TPP while analytical study investigates hypothesis regarding relationships between health issues and risk factors. While descriptive studies (TPP) answers questions of when, who, and where, analytical studies tries to answer the question of how a population is affected by a disease and why they are affected. Overall, observational studies do not manipulate any variables and often uses comparison groups for analysis; this type of study is often done in an attempt to discover the links between exposure to certain risk factors and health outcomes. Some examples of observational studies include cohort studies, case-control studies, and cross-sectional studies.
 Cohort study: participants are categorized based on exposure to disease, risk factor, or presence of a health condition and are observed over time to see if they develop symptoms of the disease
 Case-control study: Those individuals who are identified as cases (has the disease or health condition) are compared with those who don't have the disease/health condition
 Cross - sectional study: provides a "one-shot" picture of a group at a certain point in time; participants are selected based on a specific characteristic or because they belong to a certain population/group and are examined to see how the disease/health condition has affected their group.

Process of Epidemiological Investigations 
Investigating an outbreak is a very involved multi-step process which ranges from first establishing the existence of an outbreak to communicating the findings of the investigation with the scientific community as well as the general population. The following is a rough sequence of the process of these investigations.

 Establish that there is an outbreak. Epidemiologists look at data (TPP) and surveillance studies to determine if there have been similar cases, like the one being investigated, in the past or if it is a completely new type of disease or health condition.
 Prepare for field work. Once an outbreak has been established, epidemiologists take preparations, arrange materials/equipment for travel to investigate the outbreak at its place of origin and other locations where it may have spread to.
 Verify the diagnosis. Researchers review all laboratory/clinical findings and interview patients to get a better sense of what they are dealing with and to confirm their initial diagnosis of an outbreak.
 Define/identify case. Epidemiologists must come up with a precise and standard definition of what a case is or what a case looks like because that will be used to determine who is a case and who isn't.
 Descriptive epidemiology. Then next step here is to describe the outbreak in terms of time, person, and place (TPP).
 Develop a hypothesis. Epidemiologists must formulate a hypothesis about cause/risk factors of the disease, then evaluate the hypothesis and refine it as needed.
 Implement necessary control & preventative measures. This may include things like social distancing, wearing masks, frequently washing hands, as well as isolation and quarantine.
 Communicate research/investigation findings. Epidemiologists must determine which information is important and how findings will be communicated. They must also determine who the audience is that needs to know the information (is it something only health care workers need to be on the lookout for or should the general public also be made aware?)

Professions In Epidemiology 
Epidemiology is a multidisciplinary subject. Members in this field mostly includes public health care workers and scientists from related fields such as chemists, biologists, geneticists, and anthropologists. People in epidemiology may work in hospital and research settings, as well as for federal organizations such as the Centers for Disease Control and Prevention (CDC). One particular unit that may be of interest for those pursuing epidemiology is with with Epidemic Intelligence Service (EIS), a subgroup within the CDC specializing in epidemiology. Epidemic Intelligence Service officers are field workers who investigate outbreaks in the US and other countries. Their work aids in understanding causes of outbreaks and quickly stopping spread of diseases from one place to another; notably, they have contributed to helping during various pandemics in the past, such as with smallpox, polio, and Ebola.

Medical Anthropology & Epidemiology 
One particular field that has had an impact in epidemiology is anthropology. In the past, cultural anthropologists have been a very helpful resource in bridging the gap between different countries/cultures and the epidemiological investigators. They have helped and continues to help with the development and implementation of preventative/control measures in countries in a manner that will not conflict with societal beliefs or values, which may get in the way of treatment or stopping the spread of an outbreak.

Recent studies
In recent years however, medical anthropology in particular has taken on a larger role in the field of epidemiology. Medical anthropology looks at biological, social, cultural, and linguistic anthropology to understand how these factors influence health and well-being, experience and distribution of illness, as well as prevention of treatment. As with cultural anthropologists, medical anthropologists have continued to aid in the development of public health policies. In particular, they have been very helpful in providing a unique perspective on public health discourse. For example, they investigate how culture affects research studies, are able to pick up on seemingly "irrelevant" yet important small details that an epidemiologist might miss, and they are able to provide qualitative data whereas epidemiology only focuses on quantitative data. One main thing of importance here is that while epidemiologist has largely ignored cultural factors when looking at the causes of diseases/health conditions, medical anthropology has challenged this notion and contributed to the field by showing how culture and social factors play a big role in people's willingness to follow public health guidelines/interventions or even accept treatment for their illnesses.

Whereas the nature of epidemiological investigations may lead to a reductionist or limited point of view, medical anthropology provides a more holistic view of the problem and examines the issue from different angles to best understand and help the populations in need. Another critical contribution of medical anthropology has been with regard to critical qualitative data. While epidemiology is focused mainly on quantitative data and trends in health of the population, medical anthropology provides rich source of information on understanding the population's subjective experiences and providing qualitative data explaining why a particular intervention or treatment may have failed among a particular population. Their main contributions in recent years have come from modifying and helping develop epidemiological surveys by taking into account word choice and the "social suitability" of questions, so that responses would be more accurate and thereby helping to increase the validity of the questionnaires. It has also been found that medical anthropologists have played a large role in helping locals better understand the objectives of epidemiologists and thereby helped them become more receptive of their investigations. In addition to all these, one of the major contributions of medical anthropology has come from its ability to help explain health phenomenon and create and test hypothesis relating to such explanations, in a way that epidemiologists have been unable to due to their strict focus on quantitative data. In short, while epidemiology is good at understanding numerical patterns and biological causes for disease/health conditions, it is unable to fully explain all the factors that underlie certain diseases/health conditions, and this is a gap that medical anthropology has been able to pick up on and complement by providing rich qualitative data which looks at health and diseases from a holistic perspective.

Notable works
Some notable mentions of medical anthropologists who have worked and contributed to epidemiology and public health in general include Dr. Jim Kim and Paul Farmer. Dr. Jim Kim served as the president of the World Bank while Paul Farmer is a physician and medical anthropologists who focuses on infectious diseases and treatment. Both of these men co-founded the Partners in Health (PIH) program, which provided free health care to the poorest populations in countries like Haiti, Peru, and Rwanda. Other notable mentions are Amber Wutich and Alexandra Brewis, both of whom are professors and researchers at Arizona State University. They focus on health impacts caused by resource scarcity, specifically relating to lack of access to clean water in developing countries. Wutich is a director of the Global Ethnohydrology study which looks at water knowledge and management in ten countries, while Brewis researches the impact of culture on human biology.

Population-based health management 
Epidemiological practice and the results of epidemiological analysis make a significant contribution to health management
 Assess the health states and needs of a target population
 Implement and evaluate interventions
 Provide care for members of that population

Modern population-based health management is complex. Epidemiological practice and analysis is a core component. This task requires the forward looking ability to guide how a health system responds to current health issues, and how a health system can respond to future potential population health issues.

References

Additional Bibliography 

 Last JM (2001). "A dictionary of epidemiology", 4th edn, Oxford: Oxford University Press. 5th. edn (2008), edited by Miquel Porta  
 Morabia, Alfredo. ed. (2004) A History of Epidemiologic Methods and Concepts. Basel, Birkhauser Verlag. Part I.  
 Rothman, Kenneth, Sander Greenland and Timothy Lash (2008). "Modern Epidemiology", 3rd Edition, Lippincott Williams & Wilkins. ,

Related pages 
 Centers for Disease Control and Prevention
 World Health Organization

Public health