Why are infections from animals so dangerous to humans?

Zoonotic diseases: Why are infections from animals so dangerous to humans?

In the wake of the COVID-19 global pandemic, which started when the virus SARS-CoV-2 jumped from animals into humans, we ask an important question — why are infections acquired from animals so dangerous to human health?

Share on Pinterest Humans may have contracted the new coronavirus from pangolins, which are often trafficked illegally in China.

While it is not yet clear which animals were the source of the new coronavirus — was it bats? Was it pangolins? Was it both? — scientists are sure that SARS-CoV-2, the virus that causes COVID-19, originated from animals.

The numbers of confirmed COVID-19 cases across the world are staggering. According to Johns Hopkins University, hundreds of thousands of people have contracted the virus and tens of thousands of people have died.

But zoonotic diseases — that is, diseases acquired from animals — were affecting vast numbers of people across the world before COVID-19 took center stage.

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An international report from 2012, for example, informed that a total of 56 such diseases were responsible for 2.5 billion cases of illness and 2.7 million deaths across the globe each year. These illnesses included rabies, toxoplasmosis, Q fever, Dengue fever, avian influenza, Ebola, and anthrax.

Furthermore, respiratory, flu-like diseases acquired from animals wreaked havoc in the past century. The Spanish flu caused 50 million deaths in 1918, and the Hong Kong flu caused 700,000 deaths in 1968.

So, why are diseases that humans acquire from animals so dangerous? Part of it is due to our immune system. Part of it is natural selection. The specific animal that transmits the virus may also play a role.

Below, we explore some of these factors and how they intertwine.

One reason viruses from animals are so dangerous to humans is that people have no means to deal with them. Our immune system was never ‘introduced’ to these novel viruses, so it doesn’t know how to respond to the uninvited guest.

Researchers explain that most of the viruses that enter the human body are successfully destroyed by the immune system or pass through our gastrointestinal system. However, now and then, an animal virus manages to replicate within a human host.

The moment where the animal virus replicates within the body of the first human is crucial. At this critical point, the virus can mutate and evolve “under the selective constraints of the human body for the first time, adapting and improving itself for replication in this new host.”

As this occurs, the human immune system must retaliate. It needs to ‘catch up’ with the virus’ evolution and create an immune response. The human body has never been confronted with this threat before, and therefore, has no pre-existing immunity in its arsenal — so it must devise one fast.

But, this defense — part of the adaptive immune system — takes days or longer to activate. In the meantime, the virus may have already evolved to replicate faster or even escape the immune system’s retaliation.

In other words, the animal virus and the human immune system have entered an ‘arms race’ — and like with any arms race, one of the two competitors could win, or both competitors could reach a stalemate.

Medical News Today spoke to Christopher Coleman, assistant professor of Infection Immunology at the University of Nottingham in the United Kingdom, about animal viruses, human hosts, and the role of evolution and natural selection.

“[T]he general assumption,” he explained, “is that as viruses evolve to a host, they become less dangerous to that host (they want to ensure their own transmission so don’t want to rapidly kill the host before they get a chance [to replicate]).”

“This is by no means [always] true, but a virus that adapts to humans might be less dangerous in the long term because the ‘evolutionary arms race’ between virus and host has reached a sort of stalemate where neither is perfectly happy, but neither is killed off.”

Furthermore, a “virus that fully adapts to an animal host may be completely harmless to humans,” Coleman continued.

The scientist — whose main research focuses on ‘highly pathogenic human coronaviruses’ — gave examples of aggressive animal viruses within the coronavirus family. These include the “‘infectious bronchitis virus’ of chickens, ‘feline infectious peritonitis virus’ in cats, or ‘transmissible gastroenteritis virus,’ which is near 100% fatal in piglets.”

“None of these [viruses] are known to infect or cause any disease in humans,” Coleman said.

“On the other hand, a virus that evolves in animals but also has an ability to infect humans may be more deadly if and or when it infects humans.”

This may be especially true when the animals’ immune systems are very different from those of humans, or when the animals have special defense mechanisms that humans lack.

For instance, the fact that very harmful viruses such as SARS, MERS, and Ebola have all originated in bats begs the question — what do bats have that we don’t?

How can bats fly around carrying viruses that, in some cases, are extremely deadly to humans (such as Ebola), but that do not seem to harm these creatures in the slightest?

A new study, led by Cara Brook, a postdoctoral Miller Fellow at the University of California Berkeley, asked this very question. The research shows how bats’ unique immune capabilities enable them to carry and maintain a high viral load without getting sick themselves.

“[S]ome bats,” explain Brook and colleagues in their paper, “have an antiviral immune response called the interferon pathway perpetually switched on.”

“In most other mammals, having such a hyper-vigilant immune response would cause harmful inflammation. Bats, however, have adapted anti-inflammatory traits that protect them from such harm.”

This is all great news for bats, but what does it do for other mammals? Sadly, not much. The fact that bats have such good defenses means that the virus has all the encouragement it needs to replicate more quickly.

The bats’ unique immune capabilities eventually make the viruses stronger. It is like training with an outstanding competitor and getting stronger as a result.

Brook and her team carried out experiments using cell lines from two species of bats. The results showed that in “both bat species, the strongest antiviral responses were countered by the virus spreading more quickly from cell to cell.”

“This suggests that bat immune defenses may drive the evolution of faster transmitting viruses, and while bats are well protected from the harmful effects of their own prolific viruses, other creatures, including humans, are not.”

“Our immune system would generate widespread inflammation if attempting this same antiviral strategy. But bats appear uniquely suited to avoiding the threat of immunopathology,” says Brook.

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In the case of the new coronavirus, multiple theories are circulating about the specific animal that passed on SARS-CoV-2 to humans. Scientists have implicated pangolins or even snakes as possible carriers.

Pinning down specific mammals is vital because the animal can offer insights into the genetic structure of the virus and ways to tackle it. However, it is essential not to discount the possibility that the new coronavirus might have several animal sources.

Commenting on the theory that humans contracted SARS-CoV-2 from pangolins, Coleman said: “It’s as good a theory as any […] This, of course, does not mean that pangolins are the only source — it may be that there are other species.”

“For example, with ‘the original’ SARS-CoV, civet cats were the most famous species involved, but there were other small mammals infected. Also, although dromedary camels are the source of MERS-CoV, there is strong evidence that ‘other camelids’ can also be infected.”

Regardless of which animals specifically gave humans the new coronavirus, it may be more important to ask, when and where did the virus mutate?

In a recent study, researchers led by Kristian Andersen, Ph.D., an associate professor of immunology and microbiology at the Scripps Research Institute in LaJolla, CA, used the available genomic data to determine whether the origin of the new coronavirus was natural or made by humans.

Having determined that the virus is the result of natural evolution, the authors explain that depending on whether the virus adapted in its current form in animals or humans, the course of the new coronavirus pandemic could be quite different.

“[I]f SARS-CoV-2 pre-adapted in another animal species,” write the authors in the journal Nature, “then there is the risk of future re-emergence events.”

In other words, if the virus evolved to its current state in animals, then animals would continue to pass it amongst each other, and the virus could jump back into humans at any point.

Furthermore, the researchers suggest that this scenario would explain why the virus spread so quickly. Seeing that it had already developed its pathogenic traits in animals, SARS-CoV-2 was already ‘trained’ to spread and replicate quickly once it entered its first human host.

“In contrast,” write Andersen and colleagues, “if the adaptive process occurred in humans, then even if repeated [animal-to-human] transfers occur, they are unlikely to take off without the same series of mutations,” therefore minimizing the chances of another outbreak.

For now, it is impossible to know which of the two scenarios is more likely. Only time, and more research, will tell.

For live updates on the latest developments regarding the novel coronavirus and COVID-19, click here.


How risky are air travel germs?

By Bianca Seidman

September 2, 2015 / 6:17 PM / CBS News

Germs seem to hang in the recycled air on a plane and everyone has a story about sitting near a person with frightening flu symptoms. But are airplanes and airports really any germier than other places? And do the germs cause sickness?

Bacteria are certainly present on many surfaces in an airplane cabin, including some you may not have thought of. Whether or not they are dangerous depends on what kind of bugs they are, how long a person is exposed and how a particular person’s immune system handles them.

«There are bacteria everywhere. The question is whether or not any of them are specific kinds of pathogens at levels that can cause an unacceptably high risk of disease,» Dr. Charles Haas, the Betz professor of environmental engineering in at Drexel University College of Engineering, told CBS News. He has an expertise in microbial risk assessment, including Ebola.

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«Everything is dose: How much did you start off with, how much are you left with and what’s the level you’re exposed to?»

A small study conducted by the website Travelmath tested surfaces people are likely to touch during air travel. They collected samples from five different airports and four different flights on two carriers, then tested for levels of bacteria, or colony-forming units. As in a similar study of household germs in 2011, a microbiologist collected and analyzed the samples.

The Travelmath study found high bacteria levels in several places during air travel, and the site declared these the four germiest locations on a plane, on average: the seat-back tray table; the overhead air vent; the lavatory flush button; and the seatbelt buckle. In the airports, the germiest spots tested were drinking fountain buttons and bathroom stall locks.

However, the study did not say exactly what kind of bacteria were found. And that’s part of the challenge in monitoring bacteria on airplanes — they are in constant flux due to different locations, passenger populations, airline-specific cleaning procedures and other factors.

«If there’s direct evidence that there’s been a high level of contamination, a spill or soiled matter of some sort or another, when you sit down, certainly that ought to be brought to a cabin crew’s attention for cleanup,» said Haas. «There are certainly quite a number of potential agents that could exist: specific bacteria, specific viruses, specific protozoa.»

Where do bacteria thrive? Soft, porous surfaces like upholstery, as well as harder materials that don’t have natural bacterial resistance, like plastics. The surfaces most touched by human hands or other organic materials, and those less likely to be cleaned frequently are probably the worst offenders.

Travelmath noted that the kind of bacteria they found did not appear to be the most dangerous.

«The good news is that all 26 samples from airports and flights were negative for the presence of fecal coliforms such as E. coli, which can potentially be infectious,» the study said.

The pull-down tray table was one of the most bacteria-ridden areas, according to the study. The built-in tables are touched by almost everyone and everything on a plane: people, food, drinks, books, laptops and that cute little baby’s toys, which have probably been in her mouth. The average tray table tested in the study had about 2,155 colony-forming units (CFU) per square inch, more than twice as much as the second most germy surface.

The next biggest bacteria colony was found on airport water fountains. Drinking fountain buttons had 1,240 CFU per square inch.

The next three were small areas where fingers touch, but few cleansers probably follow since they wouldn’t be top of mind for cleaning crews. They had similar amounts of bacteria. Overhead air vents, airplane lavatory flush button and seatbelt buckles came in at 285, 265 and 230 CFU per square inch, respectively. Lastly, the bathroom stall locks back at the airport had 70 CFU per square inch.


But the amount of bacteria is just one part of the equation. The rest is whether the bacteria could cause sickness and how long a person is exposed to it, meaning that longer flights can present more risk.

«It really comes down to the dose you’re exposed to and whether that dose presents an unacceptably high risk or not,» said Haas. He said that in terms of the major microbes that cause serious illness like MRSA and E.coli, it would take prolonged exposure to contract any of them from touching contaminated surfaces during a flight.

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The CDC issues quarantines that don’t allow people with certain infectious diseases to travel. And they investigate any passenger reported to have serious illness during a flight. Health officials say the most common illnesses reported to them are not superbugs or deadly viruses, but often childhood diseases like measles, rubella (German measles), pertussis (whooping cough), and meningococcal disease (meningitis).

The authors of the Travelmath study believe the bacteria on planes were most likely to grow in places the cleaning crews don’t reach as often, so there may be a need for more time to clean in-between flights, especially around seating areas.

Previous studies have shown that the seat pockets and upholstery can also hang on to bacteria for months or even years. Pillows or blankets that may be shared are also easy breeding grounds, though they are becoming more uncommon on airlines.

Airborne bacteria, which this particular study didn’t consider, may be as much or more of a concern during air travel. But it may not be the hypochondriac’s nightmare it seems either. Airplane air conditioning units pull in both air from the outside and recycle what’s inside, then pass them through HEPA air purifying filters. When the airlines use HEPA filters, the air recirculated back to the cabin should be fairly clean.

As is the case in almost any setting, personal hygiene — not necessarily obsessive antibacterial wiping —might be the best defense at 35,000 feet.

«In the end, one of the best things that people can always do is practice good hand washing, use of clean utensils and so forth,» said Haas. «Soap and water does as well as anything, and if soap and water is not available, then the hand washing products could be as good, typically the alcohol-based ones.»

First published on September 2, 2015 / 6:17 PM

© 2015 CBS Interactive Inc. All Rights Reserved.


Dangerous Metaphors: How Dehumanizing Rhetoric Works

by Dr. Anna Szilagyi



Guest blogger Dr. Anna Szilagyi describes how dehumanizing rhetoric – a key hallmark of Dangerous Speech – affects our thoughts and perceptions, making violence more acceptable.

In recent times, with the global rise of populist and far-right political movements, a “family” of dehumanizing metaphors have become common worldwide. Indeed, terms that identify different groups of people as pests, deadly animals, reptiles, parasites, disease, filth, zombies, or demons, have been used more and more conveniently by powerful political actors in various parts of the world.

In 2016, to legitimize his so-called “war on drugs” — a bloody extrajudicial crackdown on alleged drug users and dealers — the President of the Philippines, Rodrigo Duterte described those who are addicted to “shabu”, the most widely available methamphetamine in the country, this way: “They are the living, walking dead. They are of no use to society anymore.” Also in 2016, amidst a gigantic, state-orchestrated public campaign against refugees and migrants in Hungary, the prime minister of the country, Viktor Orbán argued that for his nation “migration is not a solution but a problem … not medicine but a poison.” He also added: “We don’t need it and won’t swallow it.” In January 2017, it was reported that while discussing issues related to immigration, the American President, Donald Trump referred to Haiti, El Salvador, and nations in Africa as “shithole countries” during a White House meeting.

Duterte’s zombie-metaphor (“walking dead”), Orbán’s disease-metaphor (“poison”), and the filth-metaphor Trump reportedly used (“shithole countries”), are just some of the most prominent examples of a global trend toward incendiary political rhetoric that has very dangerous implications. We also see that on some occasions, politicians seemingly speak in a literal sense, yet, through unspoken but widely understood connotations, they actually activate the same metaphor-family. In 2015, for example, the leader of Poland’s right-wing populist Law and Justice party, Jaroslaw Kaczynski argued in a campaign speech that the refugees from the Middle East bring “very dangerous diseases long absent from Europe” and carry “all sorts of parasites and protozoa, which … while not dangerous in the organisms of these people, could be dangerous here.” While he could claim that he was merely pointing to potential health risks, his implication that refugees constituted a “disease” was obvious to his listeners.

In an era when mainstream politicians do not hesitate to describe various groups of people in dehumanizing terms in order to justify their agendas and policies, it is particularly important for the public to understand in what way such a language can influence human thinking and behavior. This article attempts to show how dehumanizing terms operate, by looking at the example of a recent, deeply disturbing event of political turmoil in the United States. The discourse of white supremacist protestors during the violent unrest that took place in August 2017 in the city of Charlottesville, reminds us that the role of dehumanizing metaphors in political rhetoric cannot be belittled, neither in the context of this particular upheaval, nor in any other case.

Metaphor is not mere rhetoric

Many people may assume that metaphors are used primarily by writers, poets, and public speakers to amplify their messages. However, in reality, we all use metaphors, almost every time we speak. Metaphors help us to describe one kind of experience in terms of another. For example, when we call our loved ones “honey” or “sweetie”, we use metaphors. By employing these figures of speech, we sense that we are able to better express how we think and feel about another person.

In other words, we do not simply talk in metaphors, but we also think and feel in terms of them. In fact, as two scholars, Mark Johnson and George Lakoff have highlighted, our whole thinking is metaphorical in nature.

This also means that when we come across a metaphor, it can shape our ideas strongly without us being aware of it. Considering this, it comes as little surprise that metaphors play a crucial role in political rhetoric in general, and in the language of propaganda in particular. Simply by being exposed to particular metaphors, we may, for instance, develop very hostile feelings towards specific groups of people. Metaphors that identify others as pests, deadly animals, reptiles, parasites, disease, filth, zombies, or demons obviously fall into this category.

This dehumanizing metaphor-family evokes hostility, disdain, loathing, physical disgust, and/or bodily fear in people. The terms in question can emerge both in private and political contexts. In the political realm, such metaphors encourage people to see their fellow human beings who belong to a particular group as obnoxious, disease carrying, and/or blood thirsty creatures that should be removed or isolated from the community. The targets who are described in these dehumanizing terms most often include foreign nations, ethnic and religious minorities, social classes, LGBT people, political opponents, immigrants/asylum seekers. However, depending on the local context, other groups can be caught in the cross hairs as well. As we could see above, in recent times, such metaphors were used to justify the controversial “war on drugs” not just against dealers but also small time drug users or those accused of such in the Philippines.


Importantly, these metaphors simultaneously dehumanize their targets and justify the repressive and inhumane actions that are taken against them. Indeed, they present the hostility, policy restrictions, maltreatment, human rights violations, and physical aggression to which those people targeted are often subjected to as necessary and that can be carried out according to bureaucratic procedures — naturally excluding any emotional identification with the victims.

Lessons from Charlottesville

A recent case highlights the pervasive impact that dehumanizing metaphors have on the human mind. In August 2017, hundreds of white supremacists arrived in the American college town of Charlottesville to participate in the “Unite the Right” rally and protest against the removal of a statue of the Confederate general Robert E. Lee. The demonstration quickly turned violent, as clashes broke out between the white supremacists and the anti-racist/anti-fascist counter-demonstrators who waited for them at the scene. During the demonstrations, one person was killed and many others were seriously injured after a car rammed into the anti-racist and anti-fascist counter-protestors.

The American media outlet, Vice News followed the white supremacists throughout the escalation of the events. In their documentary “Charlottesville: Race and Terror”, some of the participants of the “Unite the Right” rally speak before, during, between, and after the violent demonstrations. The documentary clearly indicates that the white supremacist speakers utilized a full-fledged rhetorical arsenal in Charlottesville to voice and fuel hatred against black and Jewish people as well as their political opponents.

Besides using other demeaning, abusive, and racist terms, some white supremacists also utilized metaphors of disease and filth to describe blacks, Jews, and those who oppose their agenda. They argued, for instance, that they are fighting against the “parasitic class of anti-white vermin” and the “anti-white, anti-American filth”. And the language use of the speakers suggests that they indeed looked at the people whom they insulted through these metaphors and other speech strategies, in non-human terms.

One speaker said: “And, at some point, we will have enough power that we will clear them from the streets forever, that which is degenerate in white countries will be removed.” This racist statement implied that the people in question represent a hygienic threat to the society that should be eliminated. The existence of these humans was reduced by the speaker to “degeneration”. This biological term refers to unstoppable deterioration that can be prevented only by the eradication of the subject.

It is also important to notice that in the previous excerpt the speaker used the passive structure (“will be removed”). This grammatical form highlights how a subject is affected by an action, and, hence, radiates impersonality. In this case, the passive structure indicates that it is not even important who “removes” the targets, the action that is required is a task that should be executed without emotions.

The way in which a white supremacist interpreted the tragic case of the counter-protestor who was killed in the car ramming incident also shows that dehumanizing metaphors fundamentally influence how people relate to others. Referring to the counter-protestors as “animals” twice, the speaker recalled the incident this way:

“So the video appears to show someone striking that vehicle, when these animals attacked him again, and he saw no way to get away from them, except to hit the gas. And sadly, because our rivals are a bunch of stupid animals who don’t pay attention, they couldn’t just get out of the way of his car and some people got hurt. And that’s unfortunate.” He also added: “I think a lot more people are going to die before we’re done here, frankly.”

The previous quotes illustrate in an exemplary fashion that dehumanizing metaphors — such as “animals” — can fundamentally hinder basic human compassion and empathy. By arguing that the deadly incident was “unfortunate”, the speaker largely belittled the significance of the car ramming. The dehumanizing metaphor (“animals”) also supported the reversal of the victim and the victimizer roles. Indeed, the speaker presented the driver of the car as the victim, and the counter-protestors — including those who were injured and the person who died — as the aggressors.

The same speaker also used the passive voice (“before we’re done here”) in the context of a dehumanizing metaphor. The passive voice, again, implies that the elimination of the opponents is a task that should be executed without emotions.


As social beings, we are conditioned to downplay and belittle verbal abuse. Indeed, the “innocence” of words is routinely imprinted in us through popular phrases and sayings which contrast verbal and physical abuse. For example, a well-known English-language rhyme, “Sticks and stones may break my bones, but words can never hurt me”, teaches children this very idea. However, this deep-seated stereotype is false. Words can hurt and may lead to the breaking of bones and worse.

Metaphors that identify other human beings as pests, deadly animals, reptiles, parasites, disease, filth, zombies, or demons, are extremely dangerous rhetorical devices. As the case of Charlottesville also shows, such metaphors have a powerful impact on human thinking. These terms have the capacity to make people believe that other human beings are repulsive and harmful creatures who do not deserve humane treatment and should be “disappeared” from the society.

Because of this, dehumanizing metaphors have played a key role in the propaganda of genocidal regimes. The Nazis and their collaborators, who systematically killed six million Jews in the Holocaust, identified their victims as “rats”, “parasites”, “vermin”, “lice”, “bacilli”, “contagion”, and “filth”. In Pol Pot’s Cambodia, where between 1975 and 1979 nearly two million innocent people were brutally killed by the Khmer Rouge regime, government propaganda represented the victims as “microbes” who must be “swept aside” and “smashed”. In 1994, in Rwanda, in just 100 days, the ethnic Hutu extremists slaughtered at least 800,000 members of the Tutsi minority community and members of the Hutu community who were seen to sympathize with them. The Hutu propaganda called the Tutsis “cockroaches” and “snakes”.

Of course, this does not mean that dehumanizing metaphors emerge exclusively in the context of mass violence and genocide. Nevertheless, it is indeed crucial for the public to be conscious of the brutal power of words that identify other human beings as pests, deadly animals, reptiles, parasites, disease, filth, zombies, or demons. In general, these metaphors support the diminishment of boundaries between verbal abuse and actual aggressive deeds, by “entitling” people to think of and act towards others inhumanely. Actually, in an ideal world, children would be taught to avoid and resist such dangerous metaphors no later than in primary school.

This post originally appeared on Talk Decoded, a blog about the power of language in politics. Header illustration by Joy Lau, from Talk Decoded. Used with permission.


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