Anshika Jain
When people hear the word “virus,” the first association is usually disease. Viruses are commonly discussed in the context of infections, outbreaks, epidemics, and illnesses, which can make it easy to overlook a remarkable biological reality: the human body is home to an enormous diversity of viruses, including many that are not known to cause disease. Together, these viral communities are known as the human virome.
The virome is an important but comparatively less understood component of the human microbiome. While microbiome research has traditionally focused heavily on bacteria, scientists now recognise that viruses are also deeply connected with the microbial ecosystems that exist throughout the body. The human virome includes viruses that infect human cells, viruses that infect bacteria and other microorganisms, and viral genetic material that may persist within microbial genomes.
Much of the human virome remains unexplored. Modern sequencing technologies have revealed enormous viral diversity, but a substantial proportion of viral sequences cannot yet be confidently assigned to known viral groups. This unexplained portion is sometimes described as viral “dark matter.”
Understanding this hidden viral ecosystem could change how scientists think about health, disease, immunity, and the microbiome. Rather than viewing every virus as an unwanted invader, researchers are beginning to investigate viruses as members of complex biological communities that interact with bacteria, human cells, and the immune system.
What Is the Human Virome?
The human virome refers broadly to the collection of viruses and virus-like genetic material associated with the human body. These viral communities can be found across different anatomical sites, including the gut, mouth, skin, respiratory tract, and other tissues.
Importantly, the virome is not a single uniform community. Viral populations differ considerably depending on where they are found. The gut contains particularly abundant viral populations, much of which consists of bacteriophages, or viruses that infect bacteria. Other body sites contain different viral communities and often have much lower viral biomass.
The composition of an individual’s virome can also vary considerably from that of another person. Age, diet, geography, environment, immune status, and other factors can influence viral communities. This individual variation makes the virome both fascinating and challenging to study.
The idea of a naturally occurring human virome does not mean that people are continuously sick from viruses. Many detected viruses appear to coexist with their hosts without causing obvious disease. The National Institutes of Health’s Human Virome Program specifically aims to characterise the “healthy” virome and understand how viruses that normally reside in humans may influence health.
Bacteriophages: The Viruses That Target Our Microbes
One of the most important discoveries in virome research is that many viruses found inside the human body do not primarily infect human cells. Instead, they infect bacteria.
These viruses are called bacteriophages, or simply phages. They are especially abundant in the gastrointestinal tract, where enormous bacterial populations provide potential hosts. Consequently, the gut virome is dominated by a diverse collection of bacteriophages.
Phages can influence bacterial populations by infecting and sometimes destroying specific bacterial cells. Others can enter a lysogenic state, in which their genetic material becomes associated with the bacterial genome and can persist while the bacterial host reproduces.
This creates a complex ecological relationship. A change in the phage population can affect bacterial communities, while changes in bacterial communities can influence which phages are able to survive and reproduce. The virome is therefore closely connected to the bacterial microbiome rather than existing as an independent biological layer.
This phage-bacteria relationship is one of the reasons scientists increasingly describe the gut as an ecosystem rather than simply an organ responsible for digestion.
The Gut Virome and Microbial Balance
The gut is currently one of the most extensively studied locations for human virome research. Recent reviews describe the gut virome as a dynamic ecosystem composed primarily of bacteriophages alongside eukaryotic viruses and other viral entities. Researchers are investigating how this ecosystem interacts with gut bacteria, host immunity, metabolism, and intestinal health.
Phages can potentially influence which bacterial populations become abundant or decline. This means they may indirectly affect the composition and function of the broader microbiome.
For example, if a phage efficiently targets a particular bacterial population, reducing that population could create opportunities for other bacteria to expand. Conversely, a bacterial community may provide the hosts necessary for particular phages to persist.
The relationship is not necessarily beneficial or harmful in a simple way. Different viral populations can have different effects, and the same interaction may have different consequences depending on the surrounding microbial and host environment.
This complexity is one reason researchers are cautious about describing the gut virome as either inherently “good” or “bad.”
Viruses and the Immune System
The immune system and the virome exist in constant interaction. The body must distinguish between different microbial signals while maintaining appropriate responses at barrier surfaces such as the intestinal tract and respiratory system.
Researchers are investigating whether viruses that reside in the body can influence immune activity. Some studies suggest that bacteriophages and other viral particles can interact with immune cells or influence inflammatory pathways. Research into gut bacteriophages, for example, has raised questions about whether phage particles can directly affect mammalian immune responses rather than acting only through their bacterial hosts.
However, the presence of a virus or viral sequence does not automatically demonstrate a harmful or beneficial effect. Many associations identified in virome studies remain correlations rather than evidence of direct causation.
Understanding these relationships will require experiments that combine virology, immunology, microbiology, and clinical research. Scientists need to determine which viral components interact with human cells, under what conditions those interactions occur, and what their biological consequences are.
The Virome Beyond the Gut
Although gut virome research receives considerable attention, viruses are associated with multiple areas of the human body. Researchers have identified viral communities in the mouth, skin, respiratory tract, and other environments.
The respiratory virome is another emerging research area. Recent research describes a diverse collection of DNA and RNA viruses, bacteriophages, and other viral entities in the respiratory system. Researchers are investigating how these communities interact with respiratory bacteria and host immunity in both health and disease.
The respiratory virome can be difficult to study because viral concentrations may be low and viral populations can change over time. These characteristics create technical challenges for researchers attempting to distinguish persistent viral communities from temporary exposure or infection.
Similar questions arise across other body sites. Scientists increasingly want to understand not simply whether viral genetic material can be detected but whether the detected viruses are active, persistent, replicating, or simply passing through.
Viral Dark Matter
One of the most intriguing aspects of virome research is the enormous amount of viral genetic information that scientists cannot yet fully identify.
When researchers sequence viral material from human samples, many sequences do not closely match previously characterised viral genomes. These unidentified sequences are often referred to collectively as viral “dark matter.”
This presents both a challenge and an opportunity. On one hand, scientists cannot easily determine the biological function of a virus when they do not know what it is or which organisms it infects. On the other hand, the unidentified sequences may represent a large reservoir of previously unknown viral diversity.
Improved sequencing, computational biology, structural biology, and laboratory cultivation could gradually reveal what these mysterious viral populations actually do.
The scale of this unexplored territory demonstrates that current descriptions of the human virome are likely incomplete.
How Scientists Study the Human Virome
Studying viruses within the human body is technically difficult. Many viruses cannot easily be grown using conventional laboratory techniques, and viral particles may occur at relatively low concentrations in some tissues.
Metagenomic sequencing has therefore become one of the most important tools in virome research. Instead of attempting to culture every virus individually, researchers can sequence genetic material from a sample and use computational methods to identify viral sequences.
Viromics has expanded the ability to study uncultivated viruses and has revealed extensive viral diversity across ecosystems. Researchers are now combining sequencing with improved computational approaches to determine viral genomes, predict hosts, examine viral genes, and study interactions between viruses and microbial communities.
However, sequencing alone has limitations. Detecting viral DNA or RNA does not necessarily prove that a virus is actively replicating. Researchers therefore increasingly combine computational predictions with laboratory experiments and culture-based approaches.
Recent work on temperate gut phages illustrates the importance of this combination. Researchers experimentally examined predicted prophages from human gut bacterial isolates and found that only a fraction of computationally predicted prophages could be induced under the tested laboratory conditions.
This demonstrates why understanding the virome requires more than simply producing increasingly large databases of viral sequences.
The Virome Changes Throughout Life
The human virome is not static. It develops and changes throughout a person’s life, beginning during early life and responding to environmental and biological conditions.
Research into infant viromes has shown that phage-bacteria interactions can change substantially during early development. These early microbial ecosystems may be influenced by factors such as diet, immune development, and environmental exposure.
As individuals age, the composition of their microbial communities can continue to change. Diet, medication, illness, geography, lifestyle, and other factors may influence the ecological conditions in which particular viruses and their microbial hosts exist.
This means that there may not be a single definition of a “normal” human virome. Instead, healthy viral ecosystems may exist in different forms across different individuals and stages of life.
Understanding this variation will be important before researchers can determine whether a particular virome pattern represents a healthy state, a temporary change, or a potential disease-associated alteration.
The Virome and Disease Research
Scientists are increasingly investigating whether changes in the human virome are associated with diseases. Research has reported associations between altered virome composition and conditions involving the gut, metabolism, immunity, and other biological systems.
However, an association does not establish causation. A disease may change the environment of the body, which then changes microbial and viral communities. Alternatively, changes in the virome could potentially contribute to disease processes. Both possibilities need to be investigated.
This distinction is particularly important because the virome is tightly connected with other biological systems. A change in phage populations could alter bacterial communities, which could then influence metabolites or immune signaling. Determining the original cause of a biological change can therefore be extremely difficult.
Future studies will need longitudinal sampling and experimental models to determine whether particular viral changes precede disease, result from disease, or simply occur alongside it.
The Virome and Antibiotic Resistance
The relationship between phages and bacterial genetics also has implications for antibiotic resistance research. Some bacteriophages can influence bacterial evolution by transferring genetic material between bacterial hosts. This process, known as horizontal gene transfer, can alter bacterial characteristics.
The relationship is complicated because not all phages carry the same types of genes, and the consequences depend heavily on the specific virus and bacterial host involved. Researchers therefore study phage genomes to understand which genetic functions they contain and how those functions may affect microbial communities.
This area is particularly relevant because the gut contains dense microbial populations in which viruses and bacteria can interact frequently. Understanding these interactions may help scientists better understand how bacterial traits evolve within complex microbial ecosystems.
Could the Virome Become a Therapeutic Target?
The discovery that bacteriophages can shape bacterial populations has naturally raised interest in therapeutic applications. Phage therapy, for example, investigates the use of bacteriophages to target particular bacterial pathogens.
Modern virome research may eventually contribute to more precise approaches in which specific phages are selected, engineered, or combined to influence bacterial communities.
Researchers are also exploring approaches such as faecal virome transplantation and other methods designed to modify viral components of microbial ecosystems. Recent reviews identify phage-based interventions and virome-based biomarkers as emerging translational areas, while emphasising that important methodological and biological questions remain.
These approaches are still an active area of research and should not be interpreted as established treatments for general health. Their future development will depend on rigorous evidence regarding safety, specificity, dosing, and long-term effects.
The Future of Human Virome Research
The next generation of virome research is likely to become increasingly integrated with other areas of biology. Scientists are already combining viral genomics with bacterial metagenomics, metabolomics, immunology, transcriptomics, and computational modelling.
This integrated approach may help researchers understand the human body as a network of interacting biological communities rather than as a collection of isolated organisms.
Artificial intelligence and advanced computational tools may also help classify previously unknown viral sequences, predict virus-host relationships, identify viral genes, and analyse large longitudinal datasets. Yet computational predictions will still need experimental validation.
The NIH Human Virome Program identifies several major unanswered questions, including how the virome varies across tissues and populations, how it develops throughout life, which host cells support viral replication, how viruses interact with immunity, and how the virome shapes the broader microbiome.
These questions indicate how early the field remains. Scientists have discovered that the human virome is extensive, but understanding what all these viruses do will require years of interdisciplinary research.
Conclusion
The human body is not simply a habitat for bacteria, fungi, and human cells. It is also home to an enormous and dynamic viral ecosystem. The human virome includes bacteriophages that interact with bacteria, viruses associated with human cells, and large numbers of viral sequences that scientists have not yet fully characterised.
The gut has emerged as one of the most important environments for virome research, largely because of its dense microbial ecosystem and abundance of bacteriophages. Researchers are investigating how these viruses influence bacterial populations, immune signaling, metabolism, and intestinal health. At the same time, studies of the respiratory tract and other body sites are revealing that viral communities extend well beyond the gut.
Perhaps the most important lesson from virome research is that the presence of a virus does not automatically mean disease. Many viruses coexist with humans without producing obvious illness, while the biological functions of numerous others remain unknown. The challenge for scientists is to distinguish harmless passengers, ecological regulators, persistent viruses, and disease-associated agents.
As sequencing technologies, viromics, artificial intelligence, laboratory cultivation, and multi-omics approaches continue to advance, researchers may gradually transform the virome from a largely unexplored component of human biology into a better understood part of health science.
The virome within us is therefore not simply a hidden collection of viruses. It is an evolving biological ecosystem that interacts with the microbiome, the immune system, and the human host. Understanding this ecosystem could provide new insights into how health is maintained, how disease develops, and how future microbiome-based medicine might be designed.