Microorganisms: The world's invisible heroes and villains

Viruses, bacteria, parasites, fungi... a basic guide to microorganisms

Microorganisms creep into our conversations practically every day, even if we don't always notice. "John didn't go to work; he has a virus," "Laura got a cut and it's infected with a horrible bacteria"… we hear phrases like this all the time.

But let's be honest: we often speak out of inertia, because we've heard it somewhere or because we "know" it's true. Do you really understand what a virus is and how it behaves? Why does it harm the human body? How is it different from a bacteria, a toxin, or a fungus?

Today we're going to tidy up this microcosm. In detail, well explained, and with examples you'll remember, so that the next time the topic comes up in conversation, you don't just have an opinion... but also a well-founded one. Let's get going.

Microorganisms

A microorganism is any tiny living thing that can only be seen with a microscope.

Bacteria, viruses, microscopic fungi, protozoa, archaea, microalgae and even non-living entities such as microbial toxins belong to this vast microscopic club.

They all populate practically every corner of the planet (and our bodies!) and, although they sometimes cause illness, they also perform essential functions for life.

Let's look at each type briefly.

Viruses

Tiny capsules of genetic material (either DNA or RNA, but never both) wrapped in a protein shell (called a capsid ) and, in some cases, an outer membrane stolen from the cell they infected. And here's the key: they're not living beings in the classic sense because they can't reproduce or generate energy on their own. They're like a USB stick with instructions for making more copies... but they need to be plugged into a computer (in this case, a living cell) to work.

Why are they so bad if they are not “alive”?

Precisely for that reason: their only "reason for existing" is to invade a cell and hijack its molecular machinery to manufacture thousands of new copies of themselves. In the process, the host cell ceases to perform its normal functions and is often destroyed. Multiply this scenario by millions of cells, and you have the picture of a viral infection.

Where does your DNA or RNA come from?

It depends on the type of virus. Some (such as herpes or adenovirus) carry DNA, which is quite stable; others (such as influenza, measles, or coronavirus) use RNA, which mutates more quickly. This genetic material is not "our own" in the sense that it does not arise from nowhere: it is the inherited imprint of previous generations of viruses, adapted and mutated to fool our defenses. Many viruses originate in other species and, after mutating, jump to humans (a process called zoonosis), as occurred with SARS-CoV-2.

How are viruses generated?

They aren't "manufactured" spontaneously. They always need a host cell to do the heavy lifting. A virus infects, introduces its genetic material, and forces the cell to become a "viral factory." When that cell can't take it anymore, it ruptures, releasing thousands of new viral particles that will infect neighboring cells. The process is so rapid that, in a matter of hours or days, you can go from a few particles to billions in your body.

Why is it so difficult to combat them?

Because they live inside our own cells while they replicate. Antibiotics work against bacteria because they attack structures that bacteria have that we don't (cell walls, certain enzymes), but viruses use our own cellular tools. If you attack those tools, you also damage your cells. Furthermore, viruses mutate very quickly, especially RNA viruses, which allows them to evade the immune system and become resistant to antivirals.

Vaccines for viruses

Vaccines are the best defense against most viral infections because they prime the immune system before the real virus appears. How do they do this? By presenting a safe version of the virus or a portion of it (for example, a protein from its capsid or envelope) so that the body generates antibodies and memory cells. This way, when the real virus arrives, the immune system already knows how to quickly identify and neutralize it.

There are several types:

• Inactivated vaccines: the virus is dead, it cannot replicate (e.g. annual flu).
• Attenuated vaccines: The virus is live but weakened, unable to cause severe disease (e.g., measles, mumps, rubella).
• Subunit vaccines: These only contain parts of the virus, such as proteins (e.g., hepatitis B).
• mRNA vaccines: These vaccines contain instructions for our cells to temporarily produce a viral protein and thus train the immune system (e.g., Pfizer and Moderna against COVID-19).

Bacteria

They are unicellular prokaryotes, meaning they have no defined nucleus or complex organelles. All their genetic information is contained in a single circular DNA chromosome, and unlike viruses, they are alive: they feed, grow, reproduce on their own (usually by binary fission), and can survive without invading another cell.

Why can they be so dangerous?

Because some have developed mechanisms to invade our tissues, produce toxins, or evade the immune system. Furthermore, they reproduce rapidly, some as often as every 20 minutes, which means an infection can multiply explosively. Some bacteria even form biofilms: sticky communities that protect them from antibiotics and defenses (such as in dental plaque or contaminated catheters).

Where does your DNA come from?

In addition to their main chromosome, many bacteria have small pieces of DNA called plasmids. These plasmids can contain genes for antibiotic resistance or toxin production, and are transmitted not only to their offspring but also between bacteria of different species through processes such as conjugation (the exchange of genetic material across a bacterial "bridge"). This rapid genetic exchange is one of the reasons why antibiotic resistance spreads so easily.

How do bacterial infections arise and spread?

It all begins when a pathogenic bacterium enters the body (through wounds, food, water, air, or direct contact). If it evades the initial defenses, it multiplies at the entry site or invades other tissues. Many infections are localized (such as a streptococcus throat infection), but others can become systemic, affecting multiple organs (such as sepsis).

Why is it so difficult to combat them?

In principle, we have a highly effective arsenal: antibiotics. These work by blocking vital functions for bacteria, such as cell wall synthesis, DNA replication, and protein production. The problem is that the overuse or misuse of antibiotics has accelerated the emergence of resistant bacteria. Strains such as methicillin-resistant Staphylococcus aureus (MRSA) or carbapenem-resistant Klebsiella pneumoniae have turned some infections into serious medical challenges. In these cases, treatment may require last-line antibiotics, combinations, or, in the worst-case scenario, no effective option available.

Are all bacteria bad?

Not at all. Most are harmless or beneficial: many live in our gut, helping us digest and produce vitamins, others live in the soil, fixing nitrogen for plants, and many more are used in industry to produce food, medicines, or clean up pollutants. Some bacteria are even environmental "superheroes": they degrade petroleum, plastics, or toxic metals.

Microscopic fungi

When we talk about fungi, we usually think of mushrooms, visible molds or yeasts for making bread, but in microbiology we also talk about microscopic fungi: eukaryotic organisms (with a nucleus and organelles) that can be unicellular, like yeasts, or multicellular, forming filaments called hyphae.

Why can they be dangerous?

Because, unlike bacteria and viruses, they share many structures and processes with our own cells (since we are eukaryotes like them). This makes them more difficult to attack without harming us. Furthermore, some species can adapt very well to our bodies, growing on skin, mucous membranes, or even internal organs, especially if the immune system is weakened. Some fungi produce highly resistant spores that survive in the environment and can infect when inhaled.

Where does your DNA come from?

As eukaryotic organisms, fungi contain their DNA in a true nucleus, distributed across several linear chromosomes. They also contain DNA in organelles such as mitochondria. Their capacity for genetic variability comes from both asexual reproduction (rapid, by budding or hyphal fragmentation) and sexual cycles that mix genetic material, allowing for adaptations and resistance to antifungals.

How do fungal infections arise and spread?

Most microscopic fungi live harmlessly in the environment or as part of our microbiota. Problems arise when immune defenses are weakened, there are alterations in the bacterial flora (for example, after antibiotics), or the skin or mucous membranes are damaged. In these cases, opportunistic species such as Candida albicans can multiply, causing candidiasis, while others such as Aspergillus can invade the lungs in immunocompromised individuals. Transmission can occur through direct contact, inhalation of spores, or internal proliferation of species already present in the body.

Why is it so difficult to combat them?

Fungi share many of our proteins and cellular structures, so antifungals must target very specific characteristics of the fungus (such as ergosterol in its membrane, equivalent to human cholesterol). This narrows the possible targets, and treatments often need to be prolonged to be effective. Furthermore, fungal spores and biofilms increase drug resistance.

Are all microscopic fungi bad?

Not at all. Many yeasts, such as Saccharomyces cerevisiae, are essential in baking, brewing, and winemaking. Other fungi produce antibiotics (such as penicillin, discovered in Penicillium notatum ), industrial enzymes, organic acids, and pharmacological compounds like cyclosporine. There are even fungi that form symbioses with plants (mycorrhizae), helping them absorb nutrients or breaking down complex pollutants.

Protozoa

They are unicellular eukaryotic microorganisms that, unlike microscopic algae, do not carry out photosynthesis and tend to behave as microscopic "predators," feeding on bacteria, other protozoa, or organic matter. Some live freely in water and soil, while others are parasites of animals and humans.

Why can they be dangerous?

Because many pathogenic protozoa are adept at invading tissues and evading the immune system. Some, like Plasmodium (malaria), have complex life cycles with several stages within the body and the vector (mosquito). Others, like Giardia lamblia, attach to the intestine, causing chronic diarrhea. Furthermore, they often survive well within cells, making treatment difficult.

Where does your DNA come from?

As eukaryotes, they possess their DNA in a nucleus organized into chromosomes. Some also have DNA in organelles like mitochondria or even in specialized structures (apicoplasts in the case of Plasmodium ). Their genetic diversity is enormous because they can reproduce both asexually and sexually, adapting to changes in the host or the environment.

How are protozoal infections generated and transmitted?

They can be transmitted through contaminated water or food (giardiasis, amebiasis), by animal vectors (mosquitoes, tsetse flies), or by direct contact with feces or fluids from an infected host. Once inside, they reproduce rapidly and can migrate to different organs depending on the species.

Why is it so difficult to combat them?

Because many live inside our cells, protected from the immune response, and their life stages are constantly changing, with different forms requiring different treatments. Furthermore, antiparasitic drugs must be selective enough not to damage our own cells (something that is complex in eukaryotes).

Are they all harmful?

No. Some protozoa live in symbiosis with animals, helping them digest plant fibers (as in the rumen of cows or termites). Others are part of plankton and are essential to aquatic food chains.

Archaea

Prokaryotic microorganisms like bacteria, but with such distinct biochemistry that they form their own domain of life. Many are famous for living in extreme conditions: very hot, very saline, very acidic, or oxygen-free waters.

Why can they be dangerous?

Actually, there's some good news here: no archaea have been identified as pathogenic to humans so far. Their interest isn't in causing disease, but rather in their unique capabilities.

Where does your DNA come from?

They have a circular chromosome and sometimes plasmids like bacteria, but their genetic and enzymatic machinery is more similar to that of eukaryotes. This makes them unique and useful in biotechnology.

How do they originate and where do they live?

They are probably one of the oldest lineages of life. They live in salt flats, geysers, underwater volcanic vents, swamps, the guts of animals, and even the open ocean. Some produce methane as waste, which is key to the global carbon cycle.

Why are they of interest to science?

Because their enzymes can withstand extreme temperatures or conditions that would destroy normal proteins. For example, Taq polymerase and other thermostable polymerases used in PCR come from thermophilic organisms.

Microscopic algae

Photosynthetic eukaryotes that are part of the phytoplankton and produce about 50% of the planet's oxygen. They include groups such as diatoms, dinoflagellates, and chlorophytes.

Why can they be dangerous?

Some species produce toxins that accumulate in shellfish and fish, causing serious poisoning in humans. So-called "red tides" are massive blooms of microalgae that color the water and can kill fish due to lack of oxygen or toxicity.

Where does your DNA come from?

They possess chromosomes in a nucleus and additional DNA in their chloroplasts, descendants of ancient symbiotic cyanobacteria.

Why are they vital?

They are the basis of the aquatic food chain, they sequester CO₂, regulate the climate, and produce useful compounds for supplements, biofuels, and cosmetics.

Are they all harmful?

No. The vast majority are harmless and essential. Without them, atmospheric oxygen and marine ecosystems would collapse.

Toxins

Poisonous substances produced by some microorganisms (bacteria, fungi, algae). They are not living, but they can be more lethal than the microbe that produces them.

Why can they be dangerous?

Because they block vital functions of our cells: they inhibit enzymes, paralyze muscles, or destroy tissue. Examples include botulinum toxin (paralysis), tetanus toxin (muscle spasms), aflatoxins (cancer), or saxitoxin (neuromuscular blockade).

Where does your DNA “come from”?

The gene for producing the toxin is found in the DNA of the producing microorganism, sometimes in plasmids or in genes acquired through horizontal transfer. The microbe expresses it and produces the toxin as a defensive weapon or to invade the host.

Why is it so difficult to combat them?

Because once the toxin is in the body, the damage can continue even if you kill the microbe. Specific antitoxins or vaccines that train the immune system to neutralize them are needed (for example, the tetanus vaccine uses a toxoid).

Are they all harmful?

In high doses, yes. But some are used for medical purposes: microdoses of botulinum toxin treat muscular dystonias, migraines, and wrinkles.

Parasites

A parasite is not a specific type of microorganism, but rather a life strategy: living off another living being (the host) by obtaining resources from it, usually causing harm. This way of life can occur in microscopic organisms (protozoa, bacteria, fungi) as well as in macroscopic organisms (worms, arthropods).

Why can they be dangerous?

Because their survival depends on exploiting their host. This involves feeding on its nutrients, destroying its tissues, or manipulating its physiology. In some cases, the damage is minor; in others, it can be lethal. Furthermore, many have developed immune evasion mechanisms that allow them to live for years inside the body without being eliminated.

Where does your DNA come from?

Like any living being, parasites have their own genetic material, adapted to their lifestyle. In parasitic protozoa like Plasmodium, the DNA includes genes for invading red blood cells; in helminths (worms), there are genes for producing molecules that "confuse" the immune system; in intracellular parasitic bacteria like Rickettsia, the genome is reduced to the bare minimum necessary for living within cells.

How are parasitic infections transmitted and generated?

It depends on the parasite. Some are transmitted by vectors (mosquitoes, flies, fleas), others by contaminated food or water, and still others by direct contact with skin or mucous membranes. Once inside, they can remain localized (for example, in the intestine) or migrate to distant organs. Many have complex life cycles with several phases in different hosts or environments.

Why is it so difficult to combat them?

Because parasites are incredibly adaptable and, as complete organisms (in the case of protozoa, fungi, and animals), they share many characteristics with our own cells. This means that antiparasitic drugs must be highly targeted to avoid harming us. Furthermore, some have "dormant" phases that are resistant to medications, requiring long or repeated treatments.

Are all parasites bad?

In practice, they almost always cause some harm, although there are cases of more balanced relationships. Some mild intestinal parasites cause no symptoms and, in certain contexts, may even modulate the immune system in a beneficial way (the "excessive hygiene" hypothesis being studied). However, the general rule is that a parasite seeks to take advantage of its host, not benefit it.

As we can see, the microscopic world is very diverse. These tiny beings have been on Earth for billions of years, have evolved in surprising ways, and coexist with us in complex interactions. Underestimating them would be a mistake: even if we can't see them, they have an enormous impact on our health, nature, and human technologies. Below, we'll explore some interesting facts and fascinating aspects of these "little giants" of life.

A brief review of microscopic curiosities

The microbial universe is full of amazing stories. Here are some interesting facts that show just how incredible these tiny creatures can be:

• Viruses that attack bacteria (bacteriophages):

  Bacteriophages, or phages, are viruses specialized in infecting bacteria, their true natural predators. Discovered before antibiotics, they attach to bacteria, inject their genetic material, and, in many cases, destroy them by lysis, bursting them from within.

  Each phage typically attacks only one bacterial species, opening the door to phage therapy: a potential alternative for combating drug-resistant bacteria. With an estimated population of 10³¹ on Earth, they are the most abundant known life form, present in oceans, soil, and in our gut, where they keep bacteria at bay.

• Archaea in extreme environments:

  Archaea are masters of survival: they thrive in hot springs at 90°C, underwater volcanic vents, hypersaline lakes, or environments as acidic as battery acid. Among them, hyperthermophiles obtain energy from chemical reactions with sulfur or metals, and halophiles dye salt flats like those in Torrevieja pink.

  Their extreme adaptations have given us key enzymes for techniques like PCR and clues about what life might be like on other planets. And it's not all exotic: many live in soils, seas, and even in our guts, aiding in digestion and producing methane.

• Oil-eating bacteria:

  When an oil spill stains the sea, all is not lost: certain marine bacteria have learned to use crude oil as a feast. Genera such as Alcanivorax and Pseudomonas include strains capable of degrading hydrocarbons and surviving using only petroleum as an energy source.

  After disasters like the Deepwater Horizon disaster in 2010, their natural populations multiplied and helped biodegrade part of the spill in a process called bioremediation. Although its action is slower than physical processes like evaporation, they ultimately eliminate residues that could persist for years.

  Inspired by them, scientists are developing biofilters and treatments using bacterial consortia to clean contaminated water and soil. They are proof that some microbes, far from being a threat, act as true "clean-up squads" for the planet.

The oddities of the microbial world seem endless. There is a fungus, Ophiocordyceps, capable of taking control of the nervous system of ants, turning them into veritable "zombies" to disperse its spores. There are social amoebas that group together to form complex multicellular structures, giant viruses that rival some bacteria in size, and bacteria like Deinococcus radiodurans , nicknamed "Conan the Bacteria", that can withstand levels of radiation that are lethal to any other living being. The microcosm is so diverse and bewildering that each new scientific discovery seems like something out of a science fiction story.

Are they all bad? Good vs. bad microbes (not all heroes wear capes)

The word "germ" often sounds like a threat, but the reality is much more nuanced: the vast majority of microorganisms don't harm us, and countless others help us every day. We live surrounded, and inhabited, by trillions of microbes; only a minority cause disease, while the vast majority either go unnoticed or perform vital functions for our health and the balance of the planet. As the microbial version of the saying goes: not all heroes wear capes... some aren't even visible. Let's look at some of these unsung heroes.

• Our bodies are home to a veritable microscopic metropolis made up of trillions of "good" bacteria, along with yeasts and archaea, which make up the human microbiome. These commensal populations are our first line of defense: they compete with pathogenic bacteria to prevent them from colonizing our tissues, produce antimicrobial compounds, keep the immune system on alert, and aid in digestion.

  Some intestinal species produce vitamins we can't produce ourselves, such as vitamin K and several B vitamins. Others ferment dietary fiber in the colon, generating fatty acids that protect intestinal health. Not all bacteria are enemies; in fact, many are essential.

  When this balance is disrupted, due to prolonged antibiotic treatment, poor diet, or infections, we experience diarrhea, an overgrowth of fungi like Candida, or an increased risk of opportunistic infections. In short, a good portion of our microbes not only coexist with us, but actively work to keep us healthy.

• Microorganisms are the very foundation of life on Earth. The tiny photosynthetic algae in plankton generate nearly half of the oxygen we breathe; without them, atmospheric O₂ levels would plummet. On land, bacteria and fungi decompose dead organic matter and recycle carbon, nitrogen, phosphorus, and other essential nutrients. Without these decomposers, the planet's surface would be a dumping ground for corpses, and nutrients would be trapped, beyond the reach of new life forms.

  Some soil bacteria, such as Rhizobium, fix nitrogen from the air into compounds that plants can assimilate, an essential step in making proteins and DNA. In the deepest oceans, entire communities depend on chemosynthetic bacteria that obtain energy from inorganic compounds in the complete absence of light.

  Microbial symbiosis also sustains many animals: cows, termites, and other herbivores can only digest cellulose thanks to gut bacteria and protozoa, and corals depend on microalgae to provide them with energy. In short, microbes don't just recycle and oxygenate the planet: they are the invisible foundation that sustains macroscopic life, including our own.

• Many of our most beloved foods exist thanks to the silent work of microorganisms. We owe our fluffy breakfast bread to yeast, which ferments the sugars in the dough, releasing CO₂ and creating its airy texture. Cheese, yogurt, kefir, kombucha, sauerkraut, salami, soy sauce, chocolate, and coffee, all depend on microbial fermentations that provide flavor, aroma, texture, and, in many cases, prolong shelf life.

  Lactic acid bacteria, for example, transform the lactose in milk into lactic acid, coagulate casein, and give yogurt and cheese their characteristic acidity. Without fermenters, our diet would be poorer and more predictable, goodbye wines, beers, cheeses, and chocolates.

  Fermentation is an ancient biotechnology that allowed food preservation long before refrigeration, and that continues to drive industries today: from vinegar and beer production to biofuel production. Every bite of blue cheese or every cup of coffee is, in reality, a tribute to those tiny, invisible artisans who transform simple raw materials into authentic pleasures.

The human microbiome: Millions of microorganisms help you live... or cause gas

Your body is a miniature ecosystem. An adult hosts as many microbial cells as a human, approximately 50/50, spread across the gut, skin, mouth, respiratory tract, and genitourinary tract. Together, this microbiota weighs between 1 and 2 kilos, and far from being intruders, most of these inhabitants are essential allies.

The intestine is their main metropolis. There, they ferment fibers and complex carbohydrates that we cannot digest, producing vitamins (K, biotin, folate), beneficial fatty acids, and compounds that nourish our cells. They also metabolize bile salts, degrade toxins, and train the immune system to differentiate friend from foe, maintaining the balance between defense and tolerance. They even produce neurotransmitters like serotonin and dopamine, influencing our mood through the gut-brain axis.

When this community becomes unbalanced (dysbiosis), problems arise: infections like Clostridioides difficile following antibiotics, metabolic disorders, allergies, or inflammatory diseases. The so-called "hygiene hypothesis" suggests that growing up in overly sterile environments can leave the immune system untrained and prone to overreaction.

And yes, they also produce gas. During the fermentation of certain foods, they release hydrogen, CO₂, or methane. In normal amounts, this is a sign of an active colon; excess, however, can indicate imbalances or intolerances.

In short, the microbiome is an invisible but vital organ: it digests, protects, and regulates. We provide it with shelter and food; it restores our health and balance. We are a "self" that, in reality, is a microscopic multitude working in unison.

Microorganisms and Technology: CRISPR, Fermentation, Biofuels, Biomining, and More

Microorganisms have not only shaped life, they are also behind some of our most innovative tools. Their natural abilities have become drivers of technology, industry, and sustainability.

Gene editing (CRISPR/Cas9)

The CRISPR/Cas9 system, today one of the most precise techniques for editing DNA, was born as a bacterial defense against viruses. It works as an archive of viral fragments that, together with the Cas9 enzyme, identifies and cuts invading sequences. Reprogrammed by Jennifer Doudna and Emmanuelle Charpentier in 2012, it allows genes to be modified in plants, animals, and humans with a precision that transformed biotechnology and earned it the Nobel Prize in Chemistry in 2020.

Industrial fermentation

Beyond bread, cheese, and beer, fermentation is now a factory for useful molecules. Microbes produce antibiotics, hormones like insulin, enzymes for detergents, organic acids, biopolymers, and biofuels. With genetic engineering, we can program them to synthesize customized compounds, from vaccines to sustainable materials.

Microbial biofuels

Oil-rich microalgae are cultivated to obtain biodiesel; yeasts ferment sugars to produce bioethanol; methanogenic bacteria and archaea transform waste into biogas. All are examples of renewable energy that doesn't compete with food crops and that harnesses CO₂, waste, or sunlight.

Biological mining (biomining)

Bacteria like Acidithiobacillus ferrooxidans dissolve minerals and release metals like copper, gold, and nickel without the need for high temperatures or harsh chemicals. This method already produces up to 15% of the world's copper and is also used to recover metals from waste and contaminated soil.

Other applications

These include water and soil bioremediation, biofertilizers, microbiota therapies, biosensors, mycelium-based building materials, and even space projects to produce oxygen or recycle waste. Even "biological computing" using reprogrammed bacteria as living logic circuits is being explored.

Iconic diseases caused by microbes

Although most microbes coexist peacefully with us, some have been, and continue to be, at the center of major health tragedies. Four examples illustrate their impact, each representing a different type of microorganism.

Tuberculosis (TB)

Caused by the bacterium Mycobacterium tuberculosis, it usually attacks the lungs and is transmitted through the air when an infected person coughs. It causes a persistent cough, low-grade fever, night sweats, weight loss, and fatigue. It is curable with specific antibiotics, but resistant strains and coinfection with HIV keep it one of the deadliest infections: in 2021, it caused 1.6 million deaths. The BCG vaccine provides partial protection, especially in children.

Malaria

Caused by protozoa of the genus Plasmodium, transmitted by the bite of female Anopheles mosquitoes. After infecting the liver, they destroy red blood cells in cycles that generate high fever, chills, and anemia. In 2023, there were 263 million cases and an estimated 600,000 deaths, mainly in sub-Saharan Africa. It is preventable and treatable, but there is still no fully effective vaccine.

Candidiasis

A fungal infection of the Candida genus, especially C. albicans. It normally thrives without causing problems, but an imbalance in the microbiota, due to antibiotics, immunosuppression, or hormonal changes, allows it to proliferate. It can be mild (oral or vaginal) or severe in immunosuppressed patients (candidemia). Antifungals are effective, although this infection demonstrates how an opportunistic organism exploits our weaknesses.

Flu (influenza)

A respiratory illness caused by influenza A, B, or C viruses. Highly contagious, it causes high fever, muscle pain, and general malaise. Its ability to mutate requires the vaccine to be reformulated each year. Although most cases are controlled with rest and early antivirals, influenza continues to cause hundreds of thousands of deaths annually and can lead to pandemics, such as the devastating one of 1918.

These examples show that, when conditions are right, pathogenic microorganisms can test all of humanity. Surveillance, research, and global cooperation remain our best defenses in this constant struggle with the microbial world.

Can we live without them? Spoiler: no.

Imagine a snap and… all the microbes on the planet disappear. At first glance, it sounds tempting: no more bacteria or viruses, goodbye to infectious diseases. But in a matter of days, the planet and we would be on the verge of collapse. Microorganisms are the invisible scaffolding that sustains life.

Without microscopic algae and cyanobacteria, oxygen production would plummet, and the air we breathe would be depleted. Without decomposing bacteria and fungi, nutrients would cease to be recycled: the soil would become impoverished, plants would die, and the food chain would collapse from the ground up. Mountains of leaves and carcasses would accumulate without decomposing, the oceans would fill with decaying organic matter, and many animals, from pollinating insects to cows and termites, would be unable to feed without their gut microbes.

We wouldn't last long either. Without gut microbiota, we would lose defenses, essential nutrients, and a trained immune system. Industries that depend on microbes, from the production of bread, cheese, coffee, and wine to antibiotics, vaccines, and insulin, would come to a standstill. Agriculture would collapse without biological nitrogen fixation, and climate change would accelerate without phytoplankton to absorb CO₂. Even organic waste and pollutants would accumulate without biodegrading.

In short: removing microbes is like removing the small parts and oil from an engine. Everything would grind to a halt. They are our ancestors, companions, and heirs; they make us sick sometimes, but they also feed us, protect us, and sustain the world we inhabit. We are half human, half microbe… and 100% dependent on them.