Nectar of the Gods

The daughters of Aegir, the Norse god of the sea and brewer to the gods of Asgard, brew ale while their father sits in the foreground. The frothy head on a pint of ale is reminiscent of sea foam over waves, likely leading to the connection between the god of the sea and ale.

The daughters of Aegir, the Norse god of the sea and brewer to the gods of Asgard, brew ale in this image from a 19th century Swedish translation of the Poetic Edda. The frothy head on a pint of ale is reminiscent of sea foam over waves, likely leading to the connection between the god of the sea and beer.

Since antiquity, civilizations the world over have reveled in the ability to make alcohol, praising their gods for sending them the heavenly nectar that makes a man brave and his companions attractive. However, it wasn’t until some years after Antoni van Leeuwenhoek discovered his animalcules that people began to suspect that it is neither Aegir nor Mayahuel they have to thank for their mead and octli, but a microbe: Saccharomyces, or, as it’s more commonly known, yeast.

For millennia, people have taken advantage of the byproducts of a process called “fermentation.” The word fermentation has its origins in the Latin word fervere, “to boil,” as a result of the observation that bubbles form in fermenting substances, as if they were boiling. Alcoholic fermentation is one of several forms of a metabolic process called “anaerobic respiration.”

In cellular biology, respiration is the process by which nutrients, including sugars, are converted into molecules that are useful sources of energy for a cell. Anaerobic respiration is respiration that occurs when oxygen is absent.

While human cells also carry out anaerobic respiration, it is a different process than yeast’s alcoholic fermentation. In a massive blow to efforts to improve cardiovascular health, it turns out this form of fermentation, “lactic acid fermentation,” ends with the production of lactate by muscle cells, not alcohol. Thus, rather than getting progressively drunker during exercise, a person is left utterly sober and with a burning sensation in their muscles.

The hijacking of alcoholic fermentation by unwitting humans began as early as 7,000 years ago, likely accidentally. A careless farmer sealed an improperly cleaned jar containing fruit or grains, and resident microbes were happy to snack on what had been stored inside. When the jar was opened, the unsuspecting consumers would have been pleasantly surprised to find a fizzy, liquid mess that made their heads buzz.

A fermentation vat at the Woodford Reserve distillery. Fermentation is kick-started with the addition of yeast to the mash. The froth at the surface of the mash is composed of bubbles of carbon dioxide produced during fermentation.

A fermentation vat at the Woodford Reserve distillery. Fermentation is kick-started with the addition of yeast to the mash. The froth at the surface of the mash is composed of carbon dioxide bubbles produced during fermentation.
Image: Ken Thomas

These happy accidents have since been refined to an art form. Even before we had any awareness of microbes’ existence, much less of the critical role they play in fermentation, early brewers, vintners, and distillers discovered how to create an ideal environment for alcohol production through extensive trial and error.

In addition to aiding in the release of sugars, heating the sugar sources and water to high temperatures before fermentation kills microbes that could compete with yeast. This microbial competition could digest the sugars into compounds that, at best, affect the alcohol’s flavor and at worst are toxic. Ideal fermentation temperatures were found for the yeast to happily produce large amounts of “good” alcohol, without gorging themselves and producing off-flavors. Even the tradition of crushing grapes for wine by stomping on them may have helped along the fermentation process by transferring microbes, including yeast, from bare feet onto the crushed fruit.

The most commonly used brewer’s, distiller’s, and vintner’s yeasts are strains of Saccharomyces cerevisiae, the same species of yeast used as a leavening agent for bread. Different strains of S. cerevisiae have been cultivated for characteristics that make them suitable for different uses. Baker’s yeast carbonates dough very quickly and produces very little alcohol, making it ideal for producing light and fluffy dough, while brewer’s yeast is less aggressive, carbonating at a slower rate and allowing for the production of greater amounts of alcohol.

Laboratory Baker's Yeast, a strain of Saccharomyces cerevisiae, grown on an agar plate.

Laboratory Baker’s Yeast, a strain of Saccharomyces cerevisiae, grown on an agar plate.
Image: Rainis Venta

By the mid-eighteenth century, it was clear that the substance that was then simply called “ferment” was required to make both bread and alcoholic drinks. However, it wasn’t yet known that this substance was a living organism. Several of the most important minds in the foundation of modern science carried out the quest to discover how fermentation took place, which led to several key discoveries in the early days of biochemistry.

Antoine Lavoisier, a chemist widely regarded as the “father of modern chemistry,” first confirmed that yeast was fermenting our alcohol. He showed that 1/3 of the sugar added to a fermentation reaction is oxidized into carbon dioxide, hence the bubbles found in fermenting liquids, while the final 2/3 is reduced to alcohol. Lavoisier further found that “ferment” was absolutely necessary to catalyze this reaction, and that it remained unchanged from start to finish.

Saccharomyces cerevisiae undergoing gemmation (upper left), as viewed by DIC microscopy.

Saccharomyces cerevisiae undergoing gemmation (upper left), as viewed by DIC microscopy.

From the microbe’s point of view, the alcohol and carbon dioxide produced during fermentation are actually harmful waste products to be discarded. As more alcohol is produced, it eventually reaches levels that are toxic to the yeast. This is why beer and wine have on average 5% and 12% alcohol contents, respectively; any higher and the yeast begins to die (spirits begin their lives with similar alcohol contents, but are distilled down to higher concentrations.)

In 1835, Charles Cagniard de la Tour took Lavoisier’s work a bit further by showing that yeast multiplies during fermentation through a process he called gemmation. This process, also called “budding,” is a form of asexual reproduction in which a cell splits itself into two exact copies of itself.

Just over twenty years later, famed microbiologist Louis Pasteur showed that not only does yeast multiply during fermentation, but also that this multiplication and the production of alcohol occur in parallel. Additionally, he showed that the yeast had to be alive for this process to occur; if he boiled the yeast before adding it, fermentation never began.


Albert Edelfelt’s 1885 portrait of the father of microbiology, Louis Pasteur.

Though these observations seem obvious to those with a modern understanding of microbiology, it was an important discovery at the time. These facts combined indicated that fermentation is a direct result of yeast replication and growth. Pasteur thus showed for the first time that not only is fermentation a process that is carried out by a microorganism, but that it is one that is required for that microorganism to live and thrive.

Pasteur went on to fully develop our current understanding of fermentation, elucidating in detail the step-by-step process of fermentation. He also was the one to coin the phrase “anaerobic respiration” after having established that fermentation only occurs in the absence of oxygen.

Though years of study have taught us that alcohol wasn’t bestowed upon us by the great god Bibulous, we should still remember that it is a gift. The next time you meet your friends out for a drink, raise your glass to Saccharomyces cerevisiae, the microbial benefactor of booze lovers everywhere. 

You Know Plenty, John Snow

Dr. Jon Snow, the Father of Epidemiology, and his impressive sideburns.

An 1856 autotype portrait of Dr. John Snow, the Father of Epidemiology, and his impressive sideburns.
Image: Wikimedia Commons

Even the most casual Game of Thrones fan knows Jon Snow, the illegitimate son of the late fan favorite, Ned Stark. As a man of the Night’s Watch, Jon Snow is charged with defending Westeros from the terrors beyond the Wall, including keeping Wildling invasions and the terrifying Others (White Walkers) at bay.

However, many of you have probably not heard of another John Snow who went down in history as a great defender of men. Now known as the Father of Epidemiology, this John Snow was a 19th century doctor and revolutionary thinker who bucked the established paradigm of then-modern thought.


In the mid-1800’s, Soho in London’s West End was not exactly the height of fashion. It was a squalid, densely populated working-class neighborhood that was largely neglected by the well-to-do of London. At this time, Soho’s residents depended on poorly maintained cesspits or the River Thames for waste disposal, rather than a well-planned sewage system.

The poor quality of Thames drinking water, depicted in William Heath's 1828 cartoon, A monster soup, commonly called Thames Water. Image: Wellcome Images

The poor quality of Thames drinking water depicted in William Heath’s 1828 cartoon, A monster soup, commonly called Thames Water.
Image: Wellcome Images

This lack of basic water and sanitation infrastructure meant that the many residents of Soho were swimming in their own filth. Accounts from this time describing cellars three feet deep in human excrement and nearly opaque drinking water the color of green tea feel ripped from Martin’s pages on Flea Bottom, with the glaring difference that these accounts are not works of fiction.

A member of both the Royal College of Surgeons of England and the Royal College of Physicians, John Snow not-of-Winterfell fought to combat poor hygiene and disease. Though he was by training an anesthesiologist (he personally administered chloroform to Queen Victoria during the birth of her two youngest children), recent cholera outbreaks in Soho attracted his attention. The end of August 1854 had brought with it a particularly bad bout of this disease, killing 127 people within the first three days of the outbreak.

A transmission electron microscopy image of the bacterium that causes cholera, Vibrio cholerae. Image: Tom Kirn, Ron Taylor, Louisa Howard - Dartmouth Electron Microscope Facility

A transmission electron microscope image of the bacterium that causes cholera, Vibrio cholerae.
Image: Tom Kirn, Ron Taylor, Louisa Howard – Dartmouth Electron Microscope Facility


Today, we know that cholera is spread through food and water contaminated with feces containing the bacterium Vibrio cholerae. In 1854, however, the idea that living, microscopic organisms cause infectious diseases, called the “Germ Theory of Disease,” would not have been accepted by the medical profession, as Louis Pasteur did not come up with it until 1861.

The “Miasma Theory of Disease” that pervaded at that time had existed since medieval times. It stated that foul, poisonous air made people sick; in fact, this idea was how the disease malaria got its name, being medieval Italian for “bad air.”

Though John Snow knew as little about infectious microbes as anyone else at the time, his observations about the patterns of the spread of disease led him to believe that bad air had nothing to do with it. He traveled from house to house, collecting data on the sick and the deceased in order to map out the source of the outbreak.

By asking questions about people’s habits and behavior, he found that the common connection between many of the sick was that their main source of drinking water was a water pump that sat on the corner of Broad Street and Cambridge Street.

ghost map

John Snow’s map of cholera cases in Soho. Each black bar represents one case of cholera. The Broad Street pump is marked in the center of the map.
Image: John Snow, M.D., On the Mode of Communication of Cholera

By the time the Board of Guardians of St. James’s Parish met on September 7th, 1854, nearly 500 people had died from the cholera outbreak that was sweeping through Soho. Firmly believing the Broad Street pump to be the center of the outbreak, John Snow testified in front of the Board, urging them to remove the pump’s handle in order to stop further spread of the disease.


Though the Board was skeptical of his claims, they took his advice and mandated that the handle be removed from the Broad Street pump to block the source of the disease. The outbreak waned shortly after the handle was removed, but few really believed Snow’s theory.

The fact was that nobody wanted to believe that the disease was being spread through drinking water contaminated with human feces; despite the obvious evidence that the water was filthy, this thought was too vile to be readily accepted. The end of the outbreak did not help his case, either – Snow was the first to admit that the outbreak had already been on the decline before they took action.

Despite this reluctance to believe Snow’s theory that disease is contagious, evidence kept piling up in his favor against the claims of those who believed in the Miasma Theory of Disease.

Afjeowi caption

George Pinwell’s 1866 cartoon, Death’s Dispensary, depicting the Broad Street Pump as the source of the 1854 Soho Cholera outbreak.
Image: CDC


Snow set out to find more convincing evidence for the connection between contaminated water and cholera infection, and discovered that those who avoided drinking from the Broad Street Pump did not become infected.

Just one block away from the Broad Street pump, workers in a local brewery and monks from a nearby monastery did not become ill, despite being surrounded by the sick. It turned out these men took a page out of the Lannister playbook; rather than drinking water, they exclusively drank alcohol, specifically the beer they brewed within their own walls.

Further investigation found that of the 535 malnourished inmates living in the terrible living conditions of a Soho workhouse, only 5 became ill. This workhouse had its own, independent source of water.

With the help of fellow contagionist Dr. Joseph J. Whiting, Snow extended his inquiries by asking residents around South London which company supplied their drinking water. They were particularly suspicious of the Southwark and Vauxhall Water Company, which could have caused cases later in the outbreak by pulling water from the Thames downstream of where cholera-contaminated sewage was dumped into the river.

A table from John Snow's 1856 paper, Cholera and the water supply in the south districts of London, 1854, showing the connection between the Southwark and Vauxhall company's water and deaths due to cholera.

A table from John Snow’s 1856 paper, Cholera and the water supply in the south districts of London, 1854, showing the connection between the Southwark and Vauxhall company’s water and deaths due to cholera.

When they compared these findings to the incidence of cholera in Soho, Snow found that 286 of the victims of the most recent outbreak drank Southwark and Vauxhall water, whereas only 14 drank water from another company that pulled water upstream of the sewage inputs (it was unknown which company provided water to the remaining 34.)

Despite the evidence in favor of John Snow’s theory, the Miasma Theory prevailed until Louis Pasteur’s development of the Germ Theory a decade later.


He may not have killed a White Walker, fought off a Wildling invasion, or even managed to convince people that disease is contagious. Still, John Snow’s impact was profound. Snow’s investigations established a new way of thinking about disease; by trying to understand human behavior and find patterns surrounding the outbreak, he laid the foundation for the field of epidemiology as we know it today.

And so – it seems it’s safe to say that in spite of the staunch opposition of the Miasmatists, you know plenty, John Snow.

Should Auld Acquaintance Be Forgot

Ms. Pelletier's American Literature and Knife-Wielding 101. Image: AMC

Ms. Peletier’s American Literature and Knife-Wielding 101.
Image: AMC

I think we can all agree that there are many drawbacks to life in the post-apocalyptic world of the Walking Dead. The lack of resources, collapse of basic infrastructure, limited medical care, and the ever-present, ravenous undead make even the simplest tasks a struggle. Let’s not even get started on traveling up Atlanta’s Downtown Connector – between the buildup of abandoned cars and flamed-out tanks, traveling through Atlanta could only be harder if there was an inch of snow on the ground.

While adjusting to the new world can be tough for many survivors, there are some surprising benefits for the children of the apocalypse: an education that includes both Tom Sawyer and knife-wielding skills, the opportunity to develop a pretty forgiving palate, and, surprisingly, an immune system that’s trained better than a Navy SEAL.

What gives little Judith Grimes a biological edge over even Michonne and her katana? Simply put, she will have grown up caked in dirt and, well…grime.


Farmer Rick’s pig, Violet, taking immune health very seriously.
Image: AMC


In the modern world, if something is dirty, you clean it. Whether your home, your body, your car, or your pet, clean is the norm and dirty should be an exception that is remedied as soon as possible. Dirt is the enemy, and cleanliness is healthy and good.

In the last quarter century, however, doctors have begun to realize that this idea is not necessarily true. Though the incidence of childhood infections has decreased since the advent of modern medicine and sanitation, more and more people are developing health problems like allergies, asthma, and autoimmune diseases.


We so often talk about threats our immune systems need to recognize and fight that we tend to forget an equally important part of the immune system’s job is to not recognize other things that are harmless. The question is, how do we teach our immune system to tell the difference between the good guys and the bad guys?

Enter what scientists call “immune tolerance.” Immune tolerance is the process by which the immune system keeps the immune cells that respond to actual threats and get rid of those that respond to things that should be ignored, like the body’s own cells.

By developing proper immune tolerance, we avoid both autoimmune diseases and chronic inflammatory diseases caused by a poorly trained, trigger-happy immune system. If the immune system constantly activates in response to things that don’t actually need to be killed, the collateral damage to the body’s own tissue is very high.

Nasty but nice - even microbes that can cause mild disease, like tapeworms, are considered "old friends" microbes. Image: Hubert Ludwig

Nasty but nice: even microbes that can cause mild disease, like this tapeworm, are considered “old friends” microbes.
Image: Hubert Ludwig


When the human immune system was first evolving back in our hunter-gatherer days, we were far from clean. People would have had regular contact with their microbial neighbors that live in the soil, water, and everywhere else, for that matter – so-called “old friends” microorganisms.

By reducing our regular contact with these “old friends,” the modern urban lifestyle that predominates in most of the developed world is associated with the uptick in both chronic inflammatory diseases, like asthma and hay fever, and autoimmune diseases, such as multiple sclerosis and Crohn’s disease.

In fact, treatment with antibiotics within the first six months of life, effectively killing off an infant’s microbes, is associated with an increased risk of such diseases. Even being born by Cesarean section increases the chances a child will develop these diseases by interfering with the colonization of the infant by its mother’s microbes during natural birth (so maybe Judith is at a slight disadvantage there…)

It appears that these interactions between humans and microbes in the dirt were so constant that as our immune system developed, it evolved a special role for these “old friends.”

Are they dirty because they're badasses, or are they badasses because they're dirty? Yes. Image: AMC

Are they dirty because they’re badasses, or are they badasses because they’re dirty? Yes.
Image: AMC


The biological reasoning behind this idea, better known as the Hygiene Hypothesis, is fairly simple.

Much like Rick’s mental state, the immune system exists in a delicate balance. One arm serves to call immune cells to action, while the other arm activates the production and release of antibodies. If one arm or the other gets too active, either autoimmune diseases or allergies can develop.

The improbable rise of autoimmune and inflammatory diseases in the 20th century puzzled scientists for some time, as they indicated problems on both arms of the immune system. How could one problem affect each arm differently from person to person?

It turns out the problem wasn’t with these different arms themselves, but with the immune cells in charge of keeping them in line. Regulatory T cells are the police force of the immune system, playing an important role in immune tolerance. When these regulatory cells find an immune cell that responds to harmless things, it hog-ties and gets rid of it before it can do more damage.

That constant contact with our “old friends” microbes gives the regulatory cells the guidebook they need to tell the Governor from good-guy Aaron. Without being able to sample these microbes and learn from them, regulatory cells don’t develop properly and can’t recognize immune cells that need to be stopped.


While proper hygiene and sanitation are very important at blocking infection with enemy microbes, they are also keeping out friendly ones that we need. In order to get around this Catch-22, many scientists are considering purposefully introducing our old friends to polite society via a different route, such as in our food or as a medication (think probiotics!)

In the meantime, our old friends will ensure little Judith and other kids rolling around in the post-apocalyptic dirt are well equipped to handle whatever the Walkers, Kirkman, or Nicotero throw at them.


Jurassic Pests

Drs. Sattler and Grant treat the sick triceratops in the film adaptation of Jurassic Park (1993).   Image:

Drs. Sattler and Grant treat the sick triceratops in the film adaptation of Jurassic Park (1993).

When considering the practical issues surrounding opening Jurassic Park, several obvious areas of concern immediately come to mind: finding an isolated chain of tropical islands, building immense electric fences, hunting versus feeding regimens, and kitchen-oriented velociraptor escape plans. Equally as important as containment of the island’s inhabitants, however, is the prevention of dino diseases that could quickly put the park out of business.

Fortunately for Jurassic Park’s veterinarians, we already have a pretty good sense of some of the major diseases that could afflict the park’s main attractions. Through careful analysis of the fossilized clues dinosaurs left behind the last time they roamed the earth, paleobiologists have discovered that the Land Before Time was crawling with the microbial ancestors of many bugs that plague tropical regions today.

A sauropod coprolite, with external surface above and cut and polished surface below.  Source: Graham Young, The Manitoba Museum

The prettiest poo you’ll ever see: a sauropod coprolite, with external surface above and cut and polished surface below. 
Image: Graham Young, The Manitoba Museum


Dinosaurs were kind enough to leave ample clues as to what plagued them in the form of coprolites – Latin for “dung stones” and English for fossilized dinosaur poo. Left behind in the coprolites are indicators that ancient forms of the very same worms and protozoa that infect humans and other modern vertebrates were also a problem for dinosaurs.

Though remains of adult parasitic worms did not survive the intervening years, fossilized eggs from trematodes, commonly known as “flatworms” or “flukes,” and three types of nematodes, or roundworms, were found in coprolites. Preserved cysts of the protozoan Entamoeba antiquus, a cousin of the modern-day gastrointestinal parasite Entamoeba histolytica, have also been seen entrapped in coprolites.

Though their eggs and cysts were shed in the dinosaur’s feces, the mature forms of all four parasites would have resided in the dinosaurs’ intestines, just like their modern-day descendants. The forms of the parasites found fossilized in the dinosaur dung are the toughest stages of these parasites’ life cycles, and help them endure the harsh environment outside their cozy host long enough to infect another individual.


Evidence of other prehistoric parasites has been found coprolites’ more popular fossil cousins, dinosaur skeletons.

For years, paleobiologists have hypothesized that lesions seen on the jawbones of Tyrannosaurus rex and its cousins were bite wounds due to fighting. However, recent investigations have shown that the lesions were actually caused by an ancestor of the protozoan Trichomonas gallinae, which is best known for causing similar disease in the beaks of modern birds.

A Tyrannosaurus rex mandible with multiple trichomonosis-type lesions (indicated by white arrows).  Image: Wolff et al., PLoS One September 2009

A Tyrannosaurus rex mandible with multiple trichomonosis-type lesions (indicated by white arrows).
Image: Wolff et al., PLoS One September 2009

In case you needed another reason to play nice with your neighbors, it turns out that the paleobiologists’ first guess actually wasn’t too far off. Though these particular bone lesions are due to disease rather than bite wounds, scientists now hypothesize that fighting and even cannibalism within tyrannosaurs were instrumental in spreading the disease.


Especially considering the park’s tropical location, of particular concern to Jurassic Park’s vets are vector-borne diseases, which are transmitted from host to host by another living organism. During the Cretaceous period (around 120 million years ago), many insects that would be familiar to us today made an appearance, bringing with them diseases that evolved to be carried by these new species.

The most prevalent vector-borne diseases are spread by blood-feeding arthropods like mosquitoes and ticks. Dinosaurs had very tough, thick hides composed of tuberculate scales, which sit next to each other but don’t overlap. Like biting insects feed off of large reptiles today, paleobiologists believed their ancestors likely fed from dinosaurs by biting the bits of skin exposed between scales.

This mosquito trapped in amber still contains the blood from its last meal in its stomach.  Source: Didier Desouens

This mosquito trapped in amber still contains the blood from its last meal in its stomach.
Image: Didier Desouens

If any bugs playing taxi to a pathogen found themselves stuck in tree sap, the fossilized sap – called amber – would freeze the bug and the contents of its gut, providing modern-day scientists with a snapshot of what that bug ate. Looking at amber-imprisoned mosquitos and sand flies under a microscope has revealed that (fortunately for Jurassic Park’s geneticists), not only did these insects feed on dinosaurs, they carried with them several familiar diseases.

Leishmania and malaria are two vector-borne protozoan parasites found in amber-preserved sand flies and mosquitoes, respectively. Today, there are a whopping 198 million cases of malaria worldwide every year, most occurring in sub-Saharan Africa; leishmania comes in behind it with 1.3 million cases annually.

Though it’s not completely clear how the disease progressed in dinosaurs, in humans, leishmania takes several different forms, from a painful, disfiguring skin disease to an often-fatal enlargement of the spleen and the liver. Malaria infects and destroys red blood cells, causing severe anemia, and, in severe cases, neurological problems and pregnancy loss. It’s likely these diseases manifested in similar ways in ancient reptiles.


With many of the diseases that burdened dinosaurs in their heyday still around today, Jurassic Park’s chief veterinarian has a lot to look out for. Fortunately for him, his charges gave him plenty of advance notice of what to expect – about 200 million years’ worth.

The Andromeda Strain

Spain's Rio Tinto is famous for its bright red hue and very acidic waters (pH 2.2). The acidity is thought result from the extremophilic bacteria living in the water. Image: Montuno, Flickr

Spain’s Rio Tinto is famous for its bright red hue and very acidic waters (pH 2.2). The acidity is thought result from the extremophilic bacteria living in the water.
Image: Montuno, Flickr

In Michael Crichton’s sci-fi thriller The Andromeda Strain, a military satellite crash-lands outside a sleepy Arizona town. After the towns’ citizens die suddenly of a mysterious illness, it becomes clear that the satellite was knocked out of orbit by a meteoroid contaminated with a deadly extraterrestrial microbe.

While this premise makes for a great story, how plausible is it? Can microbes survive a trip through space?


In order to survive space travel, a microbe would need to be very hardy.

Some microbes have the ability to go into a dormant state and shield themselves with a tough endospore. As spores, microbes can survive extreme conditions until they end up in a place where they are better suited to grow and reproduce.

Some microbes don’t just survive in harsh conditions such as very high or low temperatures, pHs, and pressures, but thrive in them. These microbes are called extremophiles, meaning “lover of extremes.” Extremophiles are found on Earth near deep-sea vents, in an asphalt lake, and in every other unlikely, inhospitable environment imaginable. Deinococcus radiodurans, for example, has no problem being pounded with gamma radiation. To each his own.

Considering the extreme conditions of outer space, extremophiles and spore-forming microbes are the most likely to survive space travel.


For an extraterrestrial pathogen to wipe out humanity, it must first find a way to leave its home planet without being killed in the process. Meteoroids from planets are released into space by an impact event, in which a very large object hits a planet with such force that it catapults pieces of that planet out of orbit.

An impact event, like this artists's depiction of a comet crashing into a planet, would cause the release of meteroids into outer space. Image: NASA

An impact event, like this artists’s depiction of a comet crashing into a planet, would cause the release of meteroids into outer space.
Image: NASA

There are three main forces associated with this kind of launch that a microbe would have to survive: acceleration, compression shock, and heating. In order to release from Mars, for example, a piece of rock would be accelerated at a rate of about 390,000 times the acceleration due to earth’s gravity, experience shock of up to 385,000 psi, and be heated to anywhere from 100 to 660 °F.

Needless to say, most of the organisms tested were killed during simulations of launch conditions. However, experimental evidence suggests that anywhere from 5 to 5 million spores of some organisms could still survive per kilogram of rock.

As this kind of impact can launch up to 1 billion kilograms of rock into space, this could still leave a very large number of living microbes to go off and wipe out intelligent life on an unsuspecting planet.


The earth is protected from cosmic rays by its magnetic field. Outside the earth's magnetosphere, however, ionizing radiation can do permanent damage to an organism's DNA. Image: NASA

The earth is protected from cosmic rays by its magnetic field. Outside the earth’s magnetosphere, cosmic radiation will not turn you into the Human Torch; it will give you cancer.
Image: NASA

The journey through outer space would prove the most arduous of the challenges facing a microbe hoping to colonize a new planet.

Outside of the protective magnetic fields that surround a planet, objects traveling through outer space are bombarded by high-energy ionizing radiation from galactic sources and the sun.

Despite comic books’ assertions to the contrary, getting hit by cosmic rays will not turn the bugs into superbugs. These stray X-rays, gamma rays, and other harmful particles damage an organism’s DNA. Though there are limits to these abilities, many extremophiles have developed ways to prevent or even repair DNA damage due to radiation.

For microbes below the surface of the meteoroid, radiation, microgravity, and extreme temperatures are not nearly as alarming as desiccation, or the lack of water caused by extreme vacuum. Without water, even the hardiest of organisms can’t perform basic processes required for it to function, leading to its slow but inexorable degradation.

Because of the dangers of desiccation, a microbe would have to take a relatively short journey if it was to survive outer space.

Austrian astronomer Edmund Weiss's 1888 depiction of the 1833 Leonid Meteor Shower.

Austrian astronomer Edmund Weiss’s 1888 depiction of the 1833 Leonid Meteor Shower.


Compared to the merciless battering it takes during launch and the bleak, unforgiving landscape of space, the forces involved in landing are relatively easy for a microbe to survive.

When a meteoroid comes close enough to Earth’s gravitational pull, it enters the upper atmosphere at speed of 10-20 km/s. Though friction heats and melts the surface of the now-meteor, as it takes less than a minute to fall to the surface of the earth, the heat doesn’t penetrate beyond the outer few millimeters of rock.

Assuming the meteor is big enough, this means the heat of re-entry would only kill microbes near the surface, which probably did not survive the journey up to this point, anyway.

The meteor is broken into fragments in the lower atmosphere and scattered over a wide area. This actually is a good thing for any microbes hoping to make Earth their new home, since it increases the chance they will land someplace nice to grow.


In order for a worldwide outbreak of the disease – a pandemic – to occur, even more chance is involved.

The extraterrestrial microbe would have to happen to land somewhere where it can both grow happily and be found by people. A contaminated meteorite that lands at the South Pole is unlikely to spark the zombie apocalypse unless the zombies in question are also penguins.

The microbe would also have to be both infectious and harmful to people. Though a microbe humans haven’t evolved to deal with is more likely to cause disease, it is far from certain that an extraterrestrial microbe would be pathogenic.


While it’s not impossible for an extraterrestrial microbe to land on Earth and begin obliterating entire towns, the sheer number of unlikely events involved makes the chances of its happening vanishingly small. An alien pandemic-Armageddon actually happening would be akin to the Universe declaring, “I hate you, Planet Earth!”