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A pandemic is a global challenge. The term is derived from the ancient Greek word pandemia, which means “belonging to all the people.” It thus refers to infectious diseases that affect the majority of the population. Pandemics often originate from pathogens transmitted from animals to humans. Our living conditions also contribute their part: worldwide trade, global mobility, or environmental damage favor the development and spread of infectious diseases. If a pandemic spreads, we have to react quickly.
A pandemic refers to the spread of a disease over entire regions, countries, and continents. If the outbreaks of the disease remain spatially and temporally limited, scientists speak of an epidemic. A pandemic is, therefore, a global epidemic.
The causes of a pandemic are pathogens that previously have either never been present in the human population or have not been present for a very long time. The human immune system is not prepared for these pathogens and the human body is therefore not sufficiently protected from the disease. These are often strains of viruses or bacteria that were previously only found in animals, for example, bird flu viruses or swine flu viruses. The genetic material of the viruses can change; the viruses mutate. Because of this they also may be transferred to humans and cause serious diseases.
Virologist Thomas Mettenleiter on species boundaries and transmission paths: “Humans are nothing special for a pathogen.” (German, English translation below)
The worldwide air transportation network: a global host mobility network. Each node represents an airport location, the links between the nodes are connections between these airports. Due to increased global mobility, the geographical distance to the initial point of an outbreak often no longer correlates with the theoretically expected time of arrival at another location in the world. Mathematical models and algorithms take into account global connectivity. Computer simulations can be used to make statistical predictions on the spatio-temporal pattern of modern, global disease dynamics. Source: Dirk Brockmann, Humboldt-Universität zu Berlin / Robert Koch-Institut
To model a disease outbreak, often the geographical distance to an initial outbreak location of a pandemic is used. The concept of effective distance takes into account the actual global connectivity. Here, a model of the H1N1 swine flu outbreak in 2009 (A) is compared to a model of the SARS outbreak in 2003 (B). Compared to the actual spread of H1N1 and SARS, models based on effective distance provided more accurate predictions of the rate of spread than models based on geographical distance. Effective distance thus allows better prediction of pandemic arrival times at further locations. Source: Dirk Brockmann, Humboldt-Universität zu Berlin / Robert Koch-Institut
The plague, for instance, is caused by a bacterium that occurs in rats and mice and is transmitted to humans via fleas. Examples of viruses are human influenza viruses, bird flu viruses, swine flu viruses, SARS coronavirus, MERS coronavirus, SARS-CoV-2 and the human immunodeficiency virus (HIV). According to the current state of knowledge, the SARS (Severe Acute Respiratory Syndrome) coronavirus was probably transmitted from bats to humans. The reservoir of the MERS (Middle East Respiratory Syndrome) coronavirus is considered to be dromedary camels (single-humped camels). These viruses were transmitted to humans either directly or through another animal species (intermediate host). The HI virus probably originates from primates. Some animal diseases, such as African swine fever, are not dangerous for humans. These pathogens cannot reproduce in humans.
Virologist Thomas Mettenleiter on animal pathogens: “There are generalists and specialists. SARS-CoV-2 is more of a generalist.” (German, English translation below)
Triggers for a pandemic can also be new virus combinations, so-called reassortants. In this case, genome fragments from at least two different virus strains mix to form a new virus. Such a virus emerges, for example, from the combination of human influenza viruses of different types or of a human influenza virus and an animal-derived virus. Pigs, in particular, can be carriers and vectors of such new combinations. They are considered a “mixing vessel”, as they can be infected with bird, human, and pig influenza viruses.
Virologist Thomas Mettenleiter on pigs as “mixing vessels” for influenza viruses: “New combinations are constantly emerging. Not only in Asia, but also in our regions.” (German, English translation below)
These pathogens can nevertheless only trigger a pandemic if they are not only transmitted from animal to human, but also from human to human. Since such viruses or bacteria are still unknown to the human immune system, the population lacks basic immunity against their infectious potential. This is what makes a pandemic without drugs and vaccines so unpredictable at first.
Scientists speak of zoonoses when pathogens are transmitted between animals and humans. The term is derived from the Greek words zoon = living being, and nosos = disease. Zoonoses can be caused by viruses, bacteria, fungi, parasites, or prions.
Classic examples of zoonoses are plague, rabies, tuberculosis, malaria, or salmonellosis. In the case of salmonellosis, the pathogen usually comes from laying hens and is transmitted via contaminated food. In humans, the pathogen then causes infectious diarrhea.
Virologist Thomas Mettenleiter on zoonoses: “Most infections in Germany are food-borne infections with salmonella.” (German, English translation below)
The conditions under which viruses cross the species barrier are still unclear. However, almost 70 percent of all infectious diseases are caused by pathogens originating from wildlife. The risk of new zoonoses has increased due to our modern living conditions. Factors such as the destruction of nature, mobility, worldwide exchange of goods or population growth favor the development and spread of zoonoses. Humans invade ecosystems, and in areas with adverse socio-economic conditions, humans and animals often live in very close proximity due to poverty. This increases the probability that pathogens spread to humans and infect them. Through global networking, international transport routes, and travel, the pathogens spread very quickly. To illustrate that human, animal, and environmental health are interdependent scientists refer to the “One Health” concept.
Virologist Thomas Mettenleiter on the One Health concept and the role of the environment: “Human, animal and environmental health are closely linked.” (German, English translation below)