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For as long as humans have existed, parasites, bacteria, and viruses have been formidable adversaries. Yet, these microscopic entities have profoundly influenced our history and evolution. They have strengthened our immune systems and played a pivotal role in shaping the planet's biological landscape. Far from being mere destructive forces, these microorganisms have contributed significantly to human progress and survival.
10. The Viruses We Carried From Africa Aided Our Survival
Advances in viral molecular genetics have revealed much about the pathogens that accompanied us on our evolutionary journey. These microbial companions have provided unexpected benefits, such as driving the development of our immune systems into their current resilient state. Viruses may have also contributed to the elimination of certain cell receptors that pathogens exploited, reducing disease susceptibility and creating a mutually beneficial relationship between humans and viruses.
These microorganisms might also have been instrumental in ensuring Homo sapiens emerged as the dominant hominid species. As our species evolved, diseases and parasites promoted genetic diversity and eliminated weaker individuals. When early humans migrated out of Africa, they carried their pathogens and parasites with them. The devastating impact of smallpox on North American and European populations is a well-documented example of this phenomenon.
Although not the sole factor, viral parasites likely spread to other hominids, such as Homo neanderthalensis (Neanderthals), who lacked prior exposure to these pathogens. Their nasal structure was less effective at filtering airborne viruses, making them more vulnerable. These diseases would have been catastrophic for other hominid species, as they were adapted to similar environments but not equipped to handle the new threats. Studies suggest that even a 2 percent higher mortality rate among Neanderthals compared to humans could have led to their extinction within 1,000 years of competition. Disease undoubtedly played a significant role in this process.
Many theories on human disease evolution suggest that these pathogens primarily developed during the Neolithic era, after humans migrated from Africa and populations grew. Evidence supports the idea of selective viral pressure during this period. Some ancient viruses were so successful that their genetic material became integrated into human DNA. For instance, the human genome contains genes from the borna virus, acquired approximately 40 million years ago. Scientists have identified around 100,000 elements of human DNA derived from viruses, largely within so-called “junk DNA.” Endogenous retroviruses, which constitute much of this DNA, are so deeply embedded in our genetic makeup that researchers have even revived one and infected hamsters and cats with it.
9. Contemporary Medical Applications of Leeches and Maggots
For millennia, the European leech (Hirudo medicinalis) has been employed in medical practices, primarily for bloodletting, addressing ailments ranging from hemorrhoids to ear infections. This tradition dates back to ancient times, with Egyptian artwork from 1500 B.C. illustrating their use. Although some cultures continued their application, Western medicine largely abandoned the practice with the advent of germ theory and a better understanding of bacteria.
However, during the 1970s and 1980s, leeches regained popularity. Surgeons specializing in cosmetic and reconstructive procedures discovered their effectiveness in reducing swelling in facial injuries, black eyes, and extremities. They are particularly useful in reattaching small body parts, such as ears or skin flaps, by preventing blood clots that could hinder recovery. Leeches have prevented amputations and even shown potential in alleviating osteoarthritis pain, with veterinarians occasionally utilizing them as well.
Maggots, meanwhile, serve as nature’s cleanup team. They excel at consuming dead or infected tissue, a process known as debridement, which exposes healthy tissue underneath. They have proven effective in treating conditions like ulcers, gangrene, skin cancer, and burns.
Despite their unsettling nature, maggots and leeches are so effective that the FDA designated them as the first “live medical devices” in 2010, fostering the growth of an industry known as biotherapy. Organizations like the Biotherapeutics Education and Research Foundation (BTERF) have emerged to promote awareness of these ancient creatures’ modern applications, and several companies now supply them for medical use.
8. Parasites and Our Immune Systems May Have Co-Evolved to Shield Us From Allergies
Scientists investigating gastrointestinal parasites have proposed a fascinating theory: After initially colonizing our digestive systems, these parasites evolved over millions of years to suppress our immune responses. Concurrently, our bodies developed mechanisms to partially counteract this suppression.
The surprising implication for human health is that, in developed nations, the widespread elimination of parasites and benign microorganisms from our bodies through modern medicine has caused our immune systems to overreact. This overcompensation has led to a rise in allergies, asthma, and eczema.
Known as the “old friends” hypothesis (often linked to the “hygiene hypothesis” but serving as a complementary theory), this idea has gained traction as we uncover more ways microorganisms have aided our survival. Clinical trials have even explored using worms to treat conditions like multiple sclerosis, inflammatory bowel disease (IBD), and allergies.
Graham A.W. Rook from University College London is the leading advocate of the old friends hypothesis. Since introducing it in 2003, the theory has also been suggested as a potential explanation for certain types of stress and depression.
Some proponents of the old friends hypothesis argue that reintroducing parasites into our bodies could address health issues caused by their absence. In 2008, John Fleming, a neurology professor at the University of Wisconsin, conducted a study where multiple sclerosis patients were infected with parasitic worms. Over three months, the average number of active brain lesions decreased from 6.6 to two. However, after the trial ended, lesions rebounded to 5.8 within two months. Earlier studies also suggested positive effects on ulcerative colitis and Crohn’s disease.
Despite its potential, parasite therapy remains experimental, with risks that may outweigh benefits. The FDA currently classifies these worms as biological products requiring safety validation before sale. Only one species, Trichuris suis, has been cleared for testing under Investigational New Drug (IND) status.
7. Virotherapy
Virotherapy, a groundbreaking medical field, involves reprogramming viruses to combat diseases. In 2005, UCLA researchers transformed a modified strain of HIV into a cancer-fighting agent, targeting and destroying cancer cells. Similarly, scientists at the Mayo Clinic in Rochester, Minnesota, engineered the measles virus for the same purpose, marking a significant advancement in biotechnology.
This method mirrors the approach used in genetically engineering plants, employing viruses as vehicles for gene delivery. Recognized as the most efficient gene transfer mechanism, this system is utilized to produce beneficial proteins in gene therapy and holds significant promise for addressing immunological conditions like hepatitis and HIV.
Since the 1950s, viruses have been acknowledged for their potential in cancer treatment, though progress was hindered by the rise of chemotherapy. Today, virotherapy demonstrates remarkable effectiveness in targeting tumors while sparing healthy cells. Clinical trials of oncolytic virotherapy have shown minimal toxicity and encouraging results. In 2013, talimogene laherparepvec (TVEC) became the first virus-based drug to achieve success in advanced-stage trials.
A major hurdle for researchers is ensuring the virus reaches its target before the immune system identifies and attacks it. Current studies focus on identifying natural tumor-targeting “carriers”—cells capable of delivering the virus without compromising their biological functions or the virus's effectiveness.
6. Harnessing Viruses to Treat Bacterial Infections
Bacteriophages, viruses that target and destroy bacteria, were first identified by Frederick Twort in 1915 and later by Felix d’Herelle. Since the 1930s, they have been instrumental in studying various aspects of viruses. These phages are particularly abundant in soil, where numerous bacterial species thrive.
Phages disrupt bacterial metabolism and eliminate them, making them potential candidates for treating a variety of bacterial infections. However, the advent of antibiotics sidelined phage therapy until the emergence of antibiotic-resistant bacteria reignited interest in this field.
Initially, the narrow specificity of phages—targeting only a limited range of bacteria or even a single species—was seen as a drawback. However, as our understanding of beneficial gut flora has grown, this specificity is now viewed as an advantage. Unlike antibiotics, which indiscriminately kill bacteria, phages can selectively target harmful bacteria without disrupting beneficial ones.
Although bacteria can develop resistance to both antibiotics and phages, new phage strains can be developed in weeks rather than years. Phages also excel at penetrating the body and locating their targets. Once the target bacteria are eliminated, phages cease reproducing and naturally die off.
5. Vaccines
Since the 1790s, when Edward Jenner created the first smallpox vaccine using cowpox, vaccines have saved millions of lives. Over time, various vaccine types have been developed. Live vaccines use weakened viruses, while inactivated vaccines contain dead pathogens or toxins, often targeting bacterial infections. Other vaccines, like subunit, conjugate, recombinant, and genetically engineered ones, utilize only specific parts of the infectious agent.
When administered, vaccines introduce a pathogen in a controlled manner, insufficient to cause illness but enough to trigger an immune response. The body learns to combat the disease, creating a memory for future encounters. Scientists have even harnessed pathogens to defend against themselves, with progress toward vaccines for certain cancers. The FDA has approved vaccines for hepatitis B (linked to liver cancer), human papillomavirus types 16 and 18 (linked to cervical cancer), and metastatic prostate cancer in some cases.
Vaccines have nearly eradicated several diseases. Smallpox is the most notable success, followed closely by polio, though it hasn’t been fully eliminated. Other diseases could be eradicated if vaccines were more accessible in underdeveloped regions. Ironically, the resurgence of these diseases is partly due to affluent, educated Westerners who reject vaccination.
The anti-vaccination movement is gaining momentum in areas where diseases were once controlled. Before the measles vaccine in 1963, the US saw 500,000 annual cases, with 500 deaths, mostly children. By 2004, cases dropped to 37, but after anti-vaccine sentiment grew, 118 cases were reported in 2011, with numbers rising due to imported cases. Whooping cough, once nearly eradicated in the US, is also making a comeback.
4. Bacterial Waste Decomposition
Among Earth's smallest and simplest organisms, bacteria play a critical role in sustaining life. Their most vital function is decomposing and recycling waste materials.
The remains of plants, animals, and organic waste contain essential nutrients and energy. Without bacteria to break these down, these resources would rapidly diminish. Fortunately, numerous bacterial species consume these materials, reducing them to basic molecules and reintroducing them into the soil, where they rejoin the ecosystem.
Beyond their natural role, humans have found numerous applications for bacteria. They are employed in sewage treatment, industrial waste processing, and cleaning up oil spills, pharmaceutical leaks, and wastewater. Bacteria also contribute to aqua-farming, algae management, and waterless sanitation systems. Researchers are exploring their potential in creating eco-friendly bioplastics, adhesives, and construction materials, as well as their ability to decompose plastic waste.
3. Cyanobacteria: The Foundation of Life as We Know It
Cyanobacteria, also known as blue-green algae, are among the oldest living organisms on Earth, with fossils tracing back billion years. These single-celled bacteria thrive in colonies and are responsible for the existence of nearly all life forms, including humans.
Cyanobacteria pioneered photosynthesis, harnessing solar energy, chemicals from ancient oceans, and atmospheric nitrogen to produce food. Their byproduct, oxygen, was initially toxic to most life forms and triggered early mass extinctions. Over 300 million years, their oxygen production helped shape Earth's atmosphere during the Archaean and Proterozoic eras.
Cyanobacteria's impact extended beyond the atmosphere. During the Proterozoic or early Cambrian era, they formed a symbiotic relationship with certain eukaryotic cells, providing food in exchange for a stable habitat. This partnership gave rise to the first plants and the mitochondria essential for animal life, marking the monumental event known as endosymbiosis.
Although some cyanobacteria are toxic, a species known as Spirulina was a staple food for the Aztecs and remains a dietary supplement in many Asian cultures. Today, it is commonly available in powder or tablet form as a health supplement.
2. Skin Bacteria: The Frontline of Immune Defense
From the moment of birth, your skin was colonized by prokaryotes and bacteria. These microorganisms, which have been with you ever since, are the result of an evolutionary partnership formed millions of years ago. Without them, survival after birth would have been impossible.
Among the most prevalent skin bacteria is Staphylococcus epidermis, which helps combat Leishmania major, the parasite responsible for leishmaniasis—a disease causing persistent skin boils and sores. This beneficial bacterium triggers an immune response called IL-1, which the body cannot produce independently, making Staphylococcus as essential to human health as any organ.
Prokaryotes, which also inhabit the digestive tract, cover every external surface of the skin. Alongside other beneficial skin microbiota, they became integral to our biology by outcompeting harmful microorganisms for space. Together with immune cells in the skin, they defend against pathogenic bacteria and invasive fungi, allowing the body to conserve energy for combating viruses and precancerous cells.
While much remains to be discovered about harnessing this knowledge for health, progress is already underway. For instance, AOBiome, a Massachusetts-based start-up, has developed a body spray containing live cultured chemoautotrophic bacteria called Nitrosomonas. The company claims the spray can “restore healthy skin bacteria” and even eliminate the need for showering, as the bacteria feed on sweat's ammonia.
1. Gut Bacteria: Essential for Survival
Once poorly understood, the bacteria residing in our gut are now recognized as vital partners to our immune system. They help expel pathogens, produce vitamin K, stimulate intestinal movement, and, most crucially, aid in digestion. Without these gut bacteria, we would lose these essential functions and face rapid mortality.
As our understanding of beneficial gut bacteria strains grows, so does our ability to integrate this knowledge into healthier lifestyles. The discovery that specific gut bacteria influence obesity has popularized probiotics, which are live bacteria found in fermented foods and sold as supplements. Certain species, such as bifidobacteria commonly present in yogurt, create acidic environments that inhibit harmful microorganisms. However, factors like fatty diets and stress can disrupt gut flora, eliminating helpful bacteria and promoting harmful ones that cause issues like gas, bloating, and leaky gut syndrome.
A groundbreaking advancement in gut bacteria research has been made by a team of Chinese and Danish scientists, who developed a novel method to identify these microorganisms using DNA sequencing. They cataloged over 500 species of beneficial bacteria and 800 new virus species that interact with them, offering potential new treatments for conditions like diabetes, obesity, and asthma.
