How do diseases spread in a population


Lutz Ehlkes

To person

Geographer, born 1983; research assistant at the Bernhard Nocht Institute for Tropical Medicine, Bernhard-Nocht-Straße 74, 20359 Hamburg. [email protected]

Jürgen May

To person

Prof. Dr. med., born 1965; Head of the "Infection Epidemiology" working group at the Bernhard Nocht Institute for Tropical Medicine. [email protected]

Swine flu, Ebola, measles - the media report almost every week on the threat of infectious diseases. The media interest often goes out as soon as the prophesied horror scenarios do not come true. The background to infection dynamics, disease risk and health interventions are rarely discussed. So why do some minor outbreaks turn into devastating epidemics while others are rapidly disappearing from the scene? The modeling, analysis and assessment of the risk potential from infectious diseases is the classic field of infection epidemiology - an interdisciplinary area that combines research and application. The teaching of epidemiology is based on the assumption that diseases do not occur purely by chance and that their occurrence, spread and spread are largely based on causal and potentially predictable factors. The main tasks of epidemiology are the quantification of the disease burden within the population, the description of the spatial and temporal occurrence of the disease and the analysis of the underlying determinants. [1]

Infectious diseases are diseases that are triggered by infectious agents - usually bacteria, viruses or parasites - or their toxic products. They are differentiated from non-communicable, such as degenerative, hereditary or mental illnesses. Infections do not necessarily go hand in hand with an illness, so they are often not immediately recognizable. Highly contagious infectious diseases are also known as "epidemics".

Infectious diseases can be endemic, epidemic and pandemic. An endemic is the constant circulation of an infection or infectious disease in the population. For example, malaria is endemic in many African countries. A temporal and spatial accumulation of cases of illness is referred to as an epidemic. This also includes disease outbreaks that are normally endemic to the population. The recurring measles outbreaks in Germany fall into this category. If such an outbreak crosses national borders and spreads indefinitely, one speaks of a pandemic. The immune disease AIDS caused by the HI virus has been spreading pandemically since the late 1980s.

The mortality and morbidity caused by infectious diseases has been decreasing in industrialized countries since the 20th century due to improved hygiene and medical innovations - such as vaccinations and antibiotics. As a result, average life expectancy is increasing and noncommunicable diseases - such as cardiovascular disease, cancer, diabetes and chronic respiratory diseases - are replacing diarrhea, measles, smallpox and tuberculosis as the main causes of death. In Germany, 16.5 times more people die from the consequences of noncommunicable diseases than from infectious diseases. [2] This trend is also evident in other affluent regions of the world.

Infectious diseases are now a major problem in poorer countries. In many regions of Africa and Asia, poor hygienic conditions prevail, especially in rural areas. There is insufficient access to medical care, and health education is often not available at all. Such conditions provide ideal conditions for infectious diseases to spread. While diseases such as HIV / AIDS have all become rare in our latitudes thanks to extensive awareness-raising campaigns and access to condoms, tuberculosis thanks to good diagnosis and therapy options, diarrheal diseases thanks to improved hygiene and measles due to broad vaccination campaigns, these diseases belong to the poorest countries in the world the main causes of death.

History of the origins of epidemiology

Hippocrates recognized a connection between the external environment and the occurrence of diseases as early as the 5th century BC. In his book "Lüfte, Gewässer, Orte". Among other things, he described the influence of weather, water quality and living conditions on physical and mental health. The basic idea of ​​epidemiology is thus as old as medicine itself. However, at that time Hippocrates suspected miasms (poisonous fumes from the soil) as the cause of infectious diseases; a theory that was still widespread into the 19th century and - despite the wrong basic assumption - saved the lives of many people through isolation and hygiene measures.

During the following 2000 years the influence of the environment on the human organism was accepted, but scientific studies on the exact effects have not been recorded. In 1662 the British haberdashery seller John Graunt compiled detailed birth and death statistics for the first time and discovered characteristic distribution patterns, broken down by gender, age and season. In the middle of the 19th century, the physician William Farr established a routine recording of deaths, which made it possible to make statistical statements about the state of health of different population groups.

A few years later, the doctor John Snow in London investigated the causes of the cholera epidemic. On the basis of the death statistics established by Farr, Snow identified areas with particularly high cholesterol mortality. He realized that the people living there got their drinking water from areas of the Thames contaminated with faeces. After the water abstraction for these areas was relocated to less polluted places on the Thames, the cholera cases decreased here in contrast to the other districts. In 1854, Snow hypothesized that - contrary to the doctrine of the time - cholera was not transmitted through miasms but through contaminated water. In order to test this hypothesis, he kept meticulous records regarding the spatial location of the cholera cases, the places where water was withdrawn (illustration 1) and - particularly noteworthy at this time - other factors that could explain the different prevalence of cholera more conclusively (e.g. age, gender, occupation, socio-economic status, size of house). On the basis of his investigations and thanks to the support of the microbiologist Arthur Hill Hassall, who was able to detect microorganisms in the drinking water and stool of the sick, he identified the water pump in Broad Street as a source of contamination and had its handle removed without further ado. The subsequent subsidence of the epidemic seemed to confirm his hypothesis. Since then, John Snow has been considered the forefather of modern epidemiology, as he was the first to combine the three central tasks of the discipline - quantification, description and analysis - with a targeted intervention. In fact, at the time of the intervention, the epidemic had already peaked and the number of cases had already declined significantly before the water pump was blocked.