Exploring the invisible world that shapes our health, evolution, and understanding of life itself
Although invisible to the naked eye, viruses are the most numerous biological entities on Earth, present in virtually all ecosystems 1 . These extraordinary infectious agents, at the frontier between the living and non-living worlds, have shaped the history of life and continue to influence our daily lives.
Viruses are typically 100 times smaller than most bacteria, requiring electron microscopy for visualization 1 .
Viruses possess either DNA or RNA as genetic material, but never both, showcasing remarkable genomic diversity 1 .
"Viruses exist at the frontier of life, challenging our definitions while demonstrating sophisticated strategies for persistence and evolution."
A virus is a submicroscopic infectious agent that can only replicate inside the living cells of an organism 1 . These acellular entities possess a single type of nucleic acid (DNA or RNA) and obligatorily parasitize a cell to ensure their reproduction 9 .
Micrometers (typical virus size)
Estimated viruses on Earth
The Baltimore classification system distinguishes seven groups of viruses based on the nature of their nucleic acid and replication strategy.
| Genetic Material Type | Virus Family | Examples | Associated Diseases |
|---|---|---|---|
| Double-stranded DNA | Herpesviridae, Poxviridae | Varicella-zoster virus, Vaccinia virus | Chickenpox, Smallpox |
| Single-stranded DNA | Parvoviridae | Parvovirus B19 | Erythema infectiosum |
| Double-stranded RNA | Reoviridae | Rotavirus | Gastroenteritis |
| Positive-sense single-stranded RNA | Coronaviridae, Picornaviridae | SARS-CoV-2, Poliovirus | COVID-19, Poliomyelitis |
| Negative-sense single-stranded RNA | Orthomyxoviridae, Rhabdoviridae | Influenza virus, Rabies virus | Influenza, Rabies |
| Single-stranded RNA with reverse transcription | Retroviridae | HIV | AIDS |
| Double-stranded DNA with reverse transcription | Hepadnaviridae | Hepatitis B virus | Hepatitis B |
Viruses originated from cellular organisms that lost unnecessary genes through parasitic lifestyle adoption 1 .
Viruses derived from DNA or RNA fragments that "escaped" from cellular organisms' genomes 1 .
Viruses evolved alongside the first cells from complex protein and nucleic acid molecules 1 .
The infectious cycle of a virus typically follows several key steps from attachment to release of new viral particles.
Recognition of specific receptors on host cell surface
Entry of virus or its genetic material into the cell
Use of cellular machinery to copy viral genetic material
Liberation of virions to infect new cells
Faced with viral invasion, the organism deploys a sophisticated defense. Recent discoveries awarded the Nobel Prize in Physiology or Medicine 2025 highlighted the crucial role of regulatory T lymphocytes (Treg) in controlling the immune response 3 .
These cells, identified by Shimon Sakaguchi and characterized through work on the Foxp3 gene by Mary Brunkow and Fred Ramsdell, act as "security guardians" of the immune system 3 . They prevent autoimmune reactions and modulate the response against pathogens.
Immune System Modulation
At the end of the 19th century, the nature of infectious agents was still mysterious. Bacteria were known, but some diseases escaped all identification. In 1892, Dmitri Ivanovsky had already observed that a pathogen of tobacco mosaic disease passed through filters that retained bacteria, but it was Martinus Beijerinck who, in 1898, understood the significance of this observation 1 5 .
Beijerinck conducted a series of rigorous experiments on tobacco mosaic disease:
The results were surprising: even after filtration, the sap from diseased plants remained infectious 1 . Beijerinck observed that the infectious agent multiplied only in the living cells of the plant and could not be cultured on artificial media like bacteria.
He named this agent "filterable virus" and described it as a "contagium vivum fluidum" (living fluid germ), emphasizing its liquid, soluble nature distinct from cellular pathogens 5 . This fundamental experiment marked the birth of virology and revealed the existence of an unsuspected biological world.
In 2025, the landscape of viral diseases continues to evolve. Respiratory infections, including COVID-19, influenza, and RSV, remain concerning, with fears of a "tripledemic" during winter months 7 .
Viruses, stripped of their pathogenic power, have become valuable tools in biotechnology and research. The Canadian Optogenetics and Vectorology Foundry at Laval University illustrates this application by producing viral vectors used to deliver genetic material into target cells 4 .
These modified viruses allow selective activation or inhibition of neurons involved in certain neurological diseases, opening new therapeutic pathways for Alzheimer's disease, spinal cord injuries, or mood disorders 4 .
Neuroscience Research
Gene Therapy
Drug Development
Viruses, although often perceived negatively as disease agents, are essential components of our living world. Their role in evolution through horizontal gene transfer, their growing use as therapeutic tools, and their place in ecosystems testify to their complexity.
Virology research continues to progress, from fundamental work on immunity to clinical applications against cancer and autoimmune diseases 3 . As new challenges emerge, as shown by recent news about SARS-CoV-2 variants, our understanding of these fascinating entities continues to deepen, paving the way for new strategies to preserve human health while appreciating the richness of the microbial world.
The next time you catch a cold, remember that you are in the presence of a master of evolution, whose history dates back to the origins of life and whose study continues to revolutionize our understanding of biology.
Beijerinck discovers viruses
First electron microscope
Hershey-Chase experiment
Polio vaccine developed
HIV identified
COVID-19 pandemic
Nobel Prize for Treg research