Epidemiology

Epidemiology is a scientific discipline in the area of public health, not just medicine, that studies the distribution, frequency, magnitude, and determinants of diseases existing in defined human populations. Rich described it in 1979 as the science that studies the dynamics of health in populations; therefore it involves the analysis and interpretation of people who are also healthy.. A professional specialized in epidemiology is called an epidemiologist.

Principles

Epidemiology —which, strictly speaking, could be called human epidemiology— constitutes a very important part of public health, occupying a special place at the intersection between biomedical sciences and medical sciences. social, and integrates the methods and principles of these sciences to study health and control diseases in well-defined human groups. There is also a veterinary epidemiology, which studies the same aspects of diseases that affect the health of animals; and one could also speak of a zoological and botanical epidemiology, closely related to ecology.

In epidemiology, diseases that occur in a certain population are studied and described, for which a series of disease patterns are taken into account, which are reduced to three aspects: time, place and person: the time it takes to arise, the season of the year in which it arises and the times in which it is most frequent; the place (city, town, country, type of area) where the cases have occurred, and the people most likely to suffer from it (children, the elderly, etc., depending on the case).

Epidemiology arose from the study of epidemics of infectious diseases; hence its name. Already in the 20th century, epidemiological studies were also extended to non-infectious diseases. For the adequate analysis of epidemiological information, a multidisciplinary team is increasingly required that includes the participation of professionals from other scientific fields, among which demography and statistics are especially important.

Science

To cause a disease, a pathogen must grow and reproduce in the host. Epidemiologists follow for this reason the natural history of pathogens. In many cases, an individual pathogen cannot grow outside of the host; if the host dies, the pathogen also dies. Likewise, pathogens that kill the host before transmitting them to another host will eventually become extinct. Therefore, most host-dependent pathogens must adapt to coexist with the host. A well-adapted pathogen lives in balance with the host, taking what it needs for its existence, and causing only minimal damage. These pathogens can sometimes cause chronic infections (infections of long duration) in the host. When there is a balance between the host and the pathogen, both survive. On the other hand, the host can be damaged when its resistance is low, by factors such as insufficient diet, old age and other stressors.

In addition, new natural pathogens sometimes emerge to which the individual host, and sometimes the entire species, has not developed resistance. These emerging pathogens often cause acute infections, characterized by a rapid and conspicuous onset. In these cases, pathogens can act as selective forces in the evolution of the host, just as the host, by developing resistance, can be a selective force in the evolution of pathogens. In cases where the pathogen is not dependent on the host for survival, the pathogen can often cause devastating acute disease.

Objectives

Epidemiology is an important part of public health and contributes to:

1. Defining the major health problems and disadvantages of a community;
2. Describe the natural history of a disease;
3. Discover the factors that increase the risk of getting a disease (their etiology);
4. To predict the trends of a disease;
5. Determine whether the disease or health problem is preventable or controlable;
6. Determine the most appropriate intervention strategy (prevention or control);
7. Prove the effectiveness of intervention strategies;
8. Quantify the benefit achieved in implementing population intervention strategies;
9. Assess intervention programmes;
10. Modern medicine, especially so-called evidence-based medicine (feual medicine or science-based medicine), is based on the methods of epidemiology.

Vocabulary

There are a number of terms that have a specific meaning to the epidemiologist. A disease is an epidemic when it occurs in an unusually high number of individuals in a population simultaneously; a pandemic is an epidemic that spreads widely, usually throughout the world.

An endemic disease is one that is constantly present in a population, although its incidence is usually low.

The incidence of a given disease is the number of new cases of an individual disease in a population in a given period of time.

The prevalence of a given disease is the total number of new and existing cases reported in a population and during a given period of time.

A disease outbreak occurs when a number of cases are observed, usually in a relatively short period of time, in a geographic area that previously had only sporadic cases of the disease.

Mortality and morbidity

Mortality is the incidence of death in the population. Infectious diseases were the leading cause of death in 1900 in developed countries, but now they are much less significant. Today, non-infectious lifestyle diseases, such as heart disease and cancer, are much more prevalent and cause higher mortality than infectious diseases. However, the current situation could change rapidly if public health infrastructures and services were to be significantly affected. In developing countries, infectious diseases are still the leading cause of mortality.

Morbidity refers to the incidence of disease in the population, including both fatal and non-fatal diseases. Morbidity statistics define the public health of a population more precisely than mortality statistics, because many diseases have relatively low mortality.

Disease progression

In terms of clinical symptomatology, the course of an acute infectious disease can be divided into stages:

1. Infection: the microorganism invades, colonizes and grows in the host.
2. Incubation period: the period of time between infection and the occurrence of symptoms of the disease.
3. Acute period: the disease is at its peak, with clear symptoms such as fever and chills.
4. Decreasing period: the symptoms of disease are ceding, the fever decreases, usually after a period of intense sweating, and a feeling of well-being appears.
5. Period of convalescence: the patient regains strength and returns to normal.

Methodology

Epidemiology is based on the scientific method to obtain knowledge, through epidemiological studies. Faced with a health problem, and the available data on it, a hypothesis is formulated, which is translated into a series of testable consequences through experimentation. A research project is then carried out that begins with data collection and its subsequent statistical analysis, which allows obtaining association measures (odds ratio, relative risk, rate ratio), effect measures (attributable risk) and impact measures (etiological fraction or proportional attributable risk), both at the level of those exposed and at the population level. From the results of this research it is possible to obtain knowledge that will serve to make public health recommendations, but also to generate new research hypotheses.

Etiology of diseases

Original Map of Dr. John Snow. Points show cases of cholera death during the London epidemic in 1854. The crossings represent the water wells of which the sick drank.

The causal epidemiological triangle of diseases is formed by the environment, the agents and the host. A change in any of these three components will alter the existing balance to increase or decrease the frequency of the disease, therefore they can be called causal or determining factors of the disease.

The foundations of modern epidemiology were laid by Girolamo Fracastoro (Verona, 1487-1573) in his works De sympathia et antipathia rerum ("On the sympathy and antipathy of things& #34;) and De contagione et contagiosis morbis, et eorum curatione ("On contagion and contagious diseases and their cure"), both published in Venice in 1546, where Fracastoro succinctly exposes his ideas on contagion and communicable diseases.

The Englishman John Graunt (1620-1674) who published in 1662 the book Natural and Political Observations Made upon the Bills of Mortality —on London— is considered one of the forerunners of epidemiology and demographics. However, it is John Snow (1813-1858), who is considered the precursor of contemporary epidemiology, since he formulated the hypothesis of the transmission of cholera by water and demonstrated it by making a map of London, where a recent outbreak epidemic had killed more than 500 people in a period of 10 days. Snow marked the homes of those who had died on the map. The distribution showed that all the deaths had occurred in the Golden Square area. The key difference between this district and the rest of London was the origin of the drinking water. The private water company that supplied the Golden Square neighborhood drew its water from a particularly polluted section of the Thames. When the water was changed and it began to be extracted upstream, from a less polluted area, the cholera epidemic subsided.

A very important development in the 20th century, published in 1956 with the results of the study of British doctors, was the demonstration of the causal relationship between smoking (smoking) and lung cancer.

Epidemiological transition

It constitutes a process of long-term dynamic change in the frequency, magnitude, and distribution of morbidity and mortality in the population.

The epidemiological transition, which is accompanied by the demographic transition, presents four aspects to highlight:

1. Displacement in the prevalence of communicable diseases by non-transmittable diseases.
2. Displacement in the morbidity and mortality of young groups to older age groups.
3. Displacement of mortality as a predominant force by morbidity, sequelae and invalidity.
4. Epidemiological polarization. Epidemiological polarization occurs when in different areas of a country or in different neighborhoods of the same city we find differences in the morbidity and mortality of the population.

Related branches

• Descriptive epidemiology: is the branch of epidemiology that describes the epidemiological in time, place and person, quantifying the frequency and distribution of the phenomenon through incidence, prevalence and mortality measures, with the subsequent formulation of hypothesis.
• Analytical epidemiology: seeks, through observation or experimentation, to establish possible causal relationships between factors to which people and populations are exposed and the diseases they present. The measures employed in the study of this branch of epidemiology are the risk factors, the result of which is a probability. It is possible to distinguish two types: absolute risk and relative risk.
  • Absolute risk: the probability of a disease (low, moderate, high); if the probability of the disease is considered for a period of time, what is being discussed is an incidence and not an absolute risk.
  • Relative risk: when comparing two absolute risks to each other; it is a relative probability (higher or lower than the other); it must be taken into account that a relative risk, however high, may be irrelevant; for example, smoking increases 100 times the risk of a disease, the risk without smoking is 1/100 000 000, so the increase for smoking is very small, practically despicable.
  • Attributable risk: in a population exposed to a risk factor, it is the difference between the incidence of disease at risk and not exposed to the risk factor. The difference between both values provides the value of the risk of disease in the exposed cohort, which is due exclusively to exposure to the risk factor.
• Experimental epidemiology: seeks, by controlling the conditions of the group to study, to draw more complex conclusions than with mere observation are not deductible. It is based on the control of subjects to study and the randomization of the distribution of individuals in two groups, an experimental group and a control group. It undertakes studies in laboratory animals and experimental studies with human populations.
• Eco-epidemiology: seeks, through ecological tools, to study comprehensively as environmental factors interact with people and populations in the media around them and as this can influence the evolution of diseases that occur as a result of such interaction.