Virus

A virus is a submicroscopic infectious particle composed of genetic material enclosed in a protein coat, capable of replicating only within the host cells of a living organism.

Viruses are found in virtually every ecosystem on Earth and are the most abundant biological entities on the planet. They are distinct from bacteria, fungi, and other microorganisms in that they lack the cellular machinery needed for independent metabolism or reproduction. Because of this dependence on host cells, viruses occupy a contested position in the definition of life — widely described as being on the boundary between living and non-living matter.

Structure

The basic structural unit of a virus is the virion, the extracellular, infectious form of the particle.

Genome

Viral genomes consist of either DNA or RNA, but never both simultaneously. The nucleic acid may be single-stranded or double-stranded, linear or circular, and can range from a few thousand to several hundred thousand base pairs in length. This genomic diversity is used as the primary basis for classifying viruses into families and orders under the International Committee on Taxonomy of Viruses (ICTV) framework.

Capsid

Surrounding the genome is a protein shell called the capsid, assembled from repeating protein subunits known as capsomers. Capsids adopt one of several geometric arrangements:

• Helical – capsomers wind in a helix around the nucleic acid (e.g., tobacco mosaic virus).
• Icosahedral – capsomers form a roughly spherical, 20-faced shell (e.g., adenovirus).
• Complex – irregular or compound structures, such as those found in bacteriophages and poxviruses.

Envelope

Many animal viruses acquire a lipid bilayer envelope derived from the host cell membrane during budding. Embedded in this envelope are glycoproteins that mediate attachment to new host cells. Enveloped viruses — such as influenza virus and HIV — are generally more susceptible to disinfectants and desiccation than non-enveloped viruses.

Replication Cycle

Viral replication proceeds through a series of defined stages, all of which depend on the host cell's biosynthetic machinery.

Attachment and Entry

The virion attaches to specific receptor molecules on the surface of the host cell. This receptor specificity largely determines the host range and tissue tropism of a given virus. After attachment, the virus enters the cell by membrane fusion, endocytosis, or direct injection of its genome (in bacteriophages).

Genome Replication and Protein Synthesis

Once inside the cell, viral genetic material is released and co-opts the host's ribosomes and metabolic resources. Depending on the type of genome, the virus may use its own RNA-dependent RNA polymerase, reverse transcriptase, or rely entirely on host DNA polymerases. New viral proteins are synthesised, and the genome is copied many times.

Assembly and Release

New virions are assembled within the host cell, incorporating freshly synthesised genomes and capsid proteins. Release occurs either by lysis — destruction of the host cell — or by budding, in which enveloped viruses gradually exit without immediately killing the cell.

Classification

Virus classification is governed by the ICTV, which recognises a hierarchical taxonomy including realm, kingdom, phylum, class, order, family, genus, and species levels. A widely used supplementary scheme is the Baltimore classification (proposed by David Baltimore in 1971), which groups viruses into seven classes according to their genome type and the strategy used to produce messenger RNA:

1. Double-stranded DNA (dsDNA) viruses
2. Single-stranded DNA (ssDNA) viruses
3. Double-stranded RNA (dsRNA) viruses
4. Positive-sense single-stranded RNA ((+)ssRNA) viruses
5. Negative-sense single-stranded RNA ((−)ssRNA) viruses
6. Reverse-transcribing RNA viruses (e.g., retroviruses)
7. Reverse-transcribing DNA viruses (e.g., hepadnaviruses)

Viruses and Disease

Viruses cause a wide spectrum of diseases in humans, animals, and plants. In humans, viral diseases range from mild infections such as the common cold (caused by rhinoviruses and coronaviruses) to life-threatening conditions such as Ebola virus disease, rabies, and AIDS. The COVID-19 pandemic (2019–present), caused by SARS-CoV-2, demonstrated the global public health impact a newly emergent virus can have.

Immune Response

The human immune system counters viral infections through both innate responses — including interferon production and natural killer cell activity — and adaptive responses involving T lymphocytes and antibody-producing B cells. Immunological memory following infection or vaccination is the basis of long-term protection against many viral diseases.

Antiviral Treatments

Unlike antibiotics for bacteria, antiviral drugs must selectively target viral processes without harming the host cell. Approved antivirals include nucleoside analogues (e.g., acyclovir for herpesviruses), protease inhibitors (used in HIV therapy), and neuraminidase inhibitors (e.g., oseltamivir for influenza). Broad-spectrum antivirals remain an active area of research.

Viruses in Evolution and Ecology

Viruses play significant roles beyond disease. Bacteriophages — viruses that infect bacteria — regulate bacterial populations in every environment, including oceans, soil, and the human gut microbiome. Horizontal gene transfer mediated by viruses contributes to the genetic diversity of microbial communities and, over evolutionary time, to the genomes of their hosts. Roughly 8% of the human genome is estimated to be of retroviral origin, consisting of endogenous retroviruses integrated during ancient infections.

Viruses are also studied as vectors in gene therapy, where modified, replication-deficient viruses deliver therapeutic genetic material into patient cells.

Frequently asked questions

Is a virus alive?

This question is genuinely debated among biologists. Viruses possess genetic information and evolve, but they lack cellular structure, independent metabolism, and the ability to reproduce without a host cell, placing them outside most standard definitions of life.

Is a virus the same as a bacterium?

No. Bacteria are single-celled living organisms with their own metabolic machinery, while viruses are non-cellular particles incapable of independent replication. Antibiotics are effective against bacteria but have no effect on viruses.

Are all viruses harmful?

Not all viruses cause disease in their hosts. Many bacteriophages infect only bacteria, some viruses are kept latent by the immune system without causing illness, and viruses play beneficial ecological roles in regulating microbial populations and driving genetic diversity.

Can vaccines prevent viral infections?

Yes, for many viruses. Vaccination stimulates the adaptive immune system to produce memory cells and antibodies, providing protection against subsequent exposure. Vaccines have eliminated or significantly reduced the burden of diseases such as smallpox, polio, and measles.

How are new viruses discovered?

New viruses are identified through clinical surveillance, environmental sampling, and increasingly through metagenomic sequencing, which allows the genetic material in a sample to be analysed without prior knowledge of what organisms are present. The pace of viral discovery has accelerated substantially since the widespread adoption of next-generation sequencing technologies.