Epidemic

Scene of the Great Plague of Seville (1649), where the crowd is shown in front of the Blood Hospital waiting for health care. In this epidemic, 46% of the inhabitants of the city died. Work located at the Hospital del Pozo Santo, Seville.

The cholera epidemic in London in 1854 was studied by John Snow who carried out this map and the first epidemiological study. Points show death cases. The crossings represent the water wells of which the sick drank.

Epidemic (from the Greek epi, for above and demos, people) is a description in community health that occurs when a disease infects to a greater than expected number of individuals in a population during a given time. To avoid the sensationalism that this word entails, the synonym of the epidemic outbreak or outbreak is sometimes used.

Currently the concept is a relationship between a baseline of a disease, which can be the normal prevalence or incidence, and the number of cases that are detected at a given time. In this sense, if a community is free of a certain disease, a single case constitutes an epidemic. In other words, it is a significantly high increase in the number of cases of a disease with respect to the number of expected cases.

If the epidemic spreads across large geographic regions on multiple continents or even around the world, it would be called a pandemic. In the case of diseases that affect an area maintained over time, it would be an endemic.

The basic reproductive rate in the simple SIR model is

R0=β β N/γ γ {displaystyle R_{0}=beta N/gamma }

where β β {displaystyle beta ,} is the infection rate (as long as one) and 1/γ γ {displaystyle 1/gamma ,} coincides with the extent of the infectious period), and the condition for the outbreak of an epidemic is:

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Etymology and meaning

Although due to its Greek etymology (demos means people) the word is originally applied to diseases that affect humans, it is also often used in everyday language to talk about diseases that affect groups of animals (zoonoses).

In fact, the proper term, epizootics, is less well known, and it is common for identical medical phenomena between animals and humans to be designated by the same words in everyday language.

Use of the term in everyday language also tends to ignore or confuse the incidence and prevalence of the disease. Thus, for example, the qualifier epidemic is often erroneously limited exclusively to cases where the prevalence is important, when there are numerous individuals who are sick regardless of their initial number, ignoring the normal incidence. The term is also used in the same way by the World Health Organization (WHO) for phenomena other than infectious diseases, such as the rapid increase in obesity on the planet.

Phases of the epidemic

Endemic, epidemic, pandemic

An endemic is the habitual presence of a disease in a geographical area (positive prevalence (stable) incidence can become an epidemic if environmental conditions allow it).

Later:

• The epidemic may spread and become a pandemic (HIV and COVID-19 case)
• The epidemic may decrease, the incidence becomes very low, zero or negative. If it stays located in space, it becomes an endemic limited to certain regions (current case of poliomyelitis). It can also disappear at the end.

Cycles and waves

Epidemics often occur in cycles or waves with a rapid boom phase and a bust, a certain period of slump, and again a boom and bust. The flu is a perfect example of a cyclical epidemic, with an annual or seasonal cycle. Epidemics can develop in one or more waves, as was the case with the 1918 flu at the turn of the XX century. Epidemiological models have shown that, under certain reasonable conditions, oscillating solutions exist, which explains why epidemics occur in cycles. For example, the common flu virus has different infection rates in winter and summer. This seasonal variability of the infection rate gives rise to a cyclical behavior (if the infection rate were constant, the simplest models lead to the conclusion that the number of infected would be a constant rate).

Development

An epidemic can also arise without a previous endemic, for example, following an accident that causes the release of a pathogen vector in an environment where it was previously non-existent (prevalence and incidence initially zero). In such circumstances, only a few cases are enough to cause a very significant increase in the incidence of the disease and give it the character of an epidemic.

Dissemination

Mark Bartholomew, a CEA researcher, and a joint CEA-CNRS-Indiana University team that has modeled the spread of epidemics using IATA databases, concluded in 2008 that "the plane is the key factor in the spread (of epidemics) throughout the world (..) The lines in which there are large flows of passengers create preferential paths for the disease. SARS reached France and Canada with flights from Hong Kong. Therefore, he believes that "even if air traffic were reduced by 90% - which seems difficult to achieve - this would limit the number of infections very little".

In animals, epidemics are spread mainly by migratory animals (see the example of bird flu).

Follow-up

In the XXI century, networks of general practitioners or hospital doctors, pharmacists and/or cities called Sentinels carry out epidemiological monitoring based on standardized protocols, at the local, regional, national and continental or even global levels in the case of diseases such as influenza.

Experts believe that emerging diseases, especially those of animal origin, will become increasingly important with population growth, urban overcrowding, climate change, increased transportation of goods and people, increased mutagenic factors and contact with new germs.

The plague of Athens (c. 1652-1654) by Michiel Sweerts, which illustrates the devastating epidemic that plagued Athens in 430 BC, as described by the historian Tucídides.

Epidemiological threshold

An epidemiological threshold, which corresponds to a minimum number of patients at the moment, is established for the main diseases, in order to compare epidemiological trends between cities, regions, countries or continents at different times.

Below this threshold, there is no talk of an epidemic. Above all, the health authorities can adopt or request preventive and precautionary measures. The number of patients at that time generally describes a bell-shaped curve.

The epidemic corresponds to the growth of an endemic disease or the appearance of a large number of patients where the disease was absent. It may also reflect the identification of a pathogen mutation that makes visible the severity of symptoms of a disease that was previously asymptomatic.

Modeling

Researchers are trying to anticipate epidemics in order to better fight them. To do this they are trying to develop and validate mathematical models.

It seems that conurbations and promiscuity alter the ecology of pathogens, allowing them to be active all year round, albeit discreetly (then referred to as "background noise").

Treatment and prevention

Camp on the outskirts of Barcelona on the occasion of the 1870 yellow fever epidemic.

Spanish doctor Jaime Ferrán developed a vaccination process for the city of Alcira (Spain) during the cholera epidemic of Valencia in 1885. Despite the success achieved, the mass inoculation system generated controversy, although it was praised by foreign doctors such as Albert Calmette and Paul Ehrlich.

The means of fighting epidemics include:

• Prevention of contagion, which historically passes through the isolation of patients (e.g. quarantine), which may be subjected to more specific measures, depending on the mode of contamination. For example, the separation of men and animals in the case of animal-borne diseases (gripe, avian influenza); the breathing mask (mascarilla) and disposable gloves against airborne diseases such as SARS or COVID-19; the condom for AIDS, decontamination (e.g., disinfection of footwear in the airports, etc.)
• Vaccination of healthy subjects
• The search for background treatment
• Preventive storage of vaccines and treatments by States.

Historical impact of epidemics

The triumph of deathPieter Brueghel oil, the Old man who allegorically describes an epidemic of plague.

Historical mentions of plagues and epidemics have been frequent throughout history. The book of Exodus describes the plagues that Moses caused to fall on Egypt, and many other Biblical references speak of epidemic outbreaks. For example, the Bible mentions the decision of Sennacherib, king of Assyria, to abandon his attempt to conquer Jerusalem in 700 B.C. C. because his soldiers were sick (Isaiah 37, 36-38).

Ancient and medieval chroniclers use the term plague to talk about such episodes, due to the ancient belief that epidemics were due to divine punishment for sinful behavior. With the advent of AIDS, various groups came to spread the idea that the disease was a punishment for immoral behavior. These visions have historically been an obstacle to the prevention and control of epidemics.

Other historical references make clear the historical impact of epidemic outbreaks and their effect on the historical course of events. The fall of empires has been directly or indirectly attributed to epidemic diseases. In the II century d. C., the calls "plagues of the reign of Antonino" (possibly measles and smallpox) swept through the Roman Empire, causing havoc and significant population decline, as well as economic hardship. This situation would have facilitated the intensification of attacks by the barbarians and finally the weakening of the empire. The Han Empire in China collapsed in the 3rd century after a series of events similar to those recorded in the Roman Empire. The defeat at the hands of Hernán Cortés of a numerically superior population of the Aztec Empire, by a few Europeans supported by their indigenous allies, was influenced by the disastrous effects that the germs of European origin had on the American populations. Smallpox not only spread through Mesoamerica, but apparently spread further south, and the weakening of the Inca Empire subjugated by Francisco Pizarro a few years later may have been an important factor. In Mexico it is estimated that the population between 1519 and 1530 fell from about 20-30 million to 2-3 million.

In Europe, the Black Death from Asia killed millions of people from its appearance in 1346 until its final eradication. Between 1346 and 1350, between a quarter and a third of the European population died from the Black Death. After that initial outbreak, there were recurring outbreaks for more than 300 years. One of the most notable outbreaks was the Great Plague of London (1665-66) or the Italian Plague (1629-1631). Because the plague struck some regions but not others, the various outbreaks of the plague had a noticeable effect on the political and economic development of Europe since the Middle Ages. In the last bubonic plague in France (1720-22), half of the population of Marseille died, 60% of Toulon, 44% of Arles and 30% of Aix-en- Provence and Avignon, although the outbreak had not spread outside of Provence.

Historian W. H. McNeill argues, in his Plagues and Peoples, that the spread of infectious diseases has been an important factor in historical development. For example, there was a marked increase in population during the 18th century, China's population rose from 150 million in 1716 to 313 million in 1794 and the population of Europe grew from 118 million in 1700 to 187 million in 1800. While there were many factors involved in this increase, including changes in the age of marriage and technological improvements that led to increased supplies, these factors would not be important in explaining by themselves such increases. Demographic studies indicate that a satisfactory explanation requires recognition of decreased mortality due to epidemic episodes.