What is a virus, and how does it work?
Explore how a package of genetic instructions uses a host cell to make new virus particles, with influenza A as an example.
Reviewed 2026-10-23
How can a virus make more of itself?
A virus is a DNA or RNA package, not a cell. It needs suitable host-cell machinery to produce a new generation.
Constructed virion cutaway and separately magnified animal-cell specimen. Particle shapes and protective proteins vary; this is not an influenza portrait.
Notes and sources for this step
What we mean by virus
This guide concerns biological viruses. A virus is an infectious agent whose genetic material is DNA or RNA and whose reproduction depends on host-cell components. The host is the organism it infects; a host cell is a cell it uses. Viruses infect humans and other animals, plants, bacteria, and other organisms. Source: National Human Genome Research Institute, Virus — https://www.genome.gov/genetics-glossary/Virus
Our practical question and scope
How can something that is not a cell make more of itself inside us? We follow the parts of a virus particle and the tasks in one successful cell infection. Influenza A supplies a concrete example. This is a conceptual explanation, not a guide to diagnosis or treatment. It does not settle the debate about whether viruses count as alive.
Instructions travel in protective packaging.
A genome carries genetic instructions; a capsid is protective protein packaging. Some virus particles also have a lipid envelope.
Generic constructed cutaway. The outer envelope can be removed; model seams are not pores. Protective-protein organization and particle shape vary among viruses.
Notes and sources for this step
Names for the parts
Genome means the genetic material. Capsid means the protective protein coat. A complete virus particle is called a virion. An envelope is a lipid, or fatty, membrane acquired from host-cell membranes and containing viral proteins. Particles differ in shape and organization; the cover illustrates an enveloped virus, not every virus. Some particles also carry enzymes needed early in infection. Source: Medical Microbiology, Structure and Classification of Viruses — https://www.ncbi.nlm.nih.gov/books/NBK8174/
A package is not a miniature cell
A virus particle lacks the independent cellular machinery needed to reproduce. Its genome does not turn the particle into a self-sufficient cell. Source: Molecular Biology of the Cell, Cell Biology of Infection — https://www.ncbi.nlm.nih.gov/books/NBK26833/
The cell supplies essential working parts.
Ribosomes are cell structures that build proteins from RNA messages. Viruses need host ribosomes and resources, alongside virus-specific enzymes and instructions.
The same cell specimen highlights ribosomes. Its nucleus is a structural landmark, not a claim that every virus copies its genome there.
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What a ribosome does
Ribosomes are the cell’s protein-making structures. They read messenger RNA to join amino acids into proteins. Viruses depend on cellular ribosomes for this work. How the viral genome is copied and how messenger RNA is made depend on the virus: viral enzymes may carry out essential steps. The cell does not necessarily do every task. Source: Molecular Biology of the Cell, Cell Biology of Infection — https://www.ncbi.nlm.nih.gov/books/NBK26833/
Why suitable matters
Attachment to a cell is only one requirement. The virus must also gain access and find conditions that support later replication steps. A surface match alone does not guarantee a productive infection. Source: Medical Microbiology, Multiplication — https://www.ncbi.nlm.nih.gov/books/NBK8181/
Step through one successful infection.
Choose a task to trace how entry, cell-supported production, assembly, and release lead to new particles.
A cell-centered map of tasks, not a universal physical pathway or timetable. Entry, assembly and release mechanisms vary by virus.
Notes and sources for this step
How to read the task map
The cell boundary locates attachment and access, and ribosomes locate the cell’s protein-making contribution. Following a task changes the relevant pathway and readout. The visible components form an explanatory map; their simultaneous presence is not a claim about molecular timing, quantities or one universal replication pathway.
Different viruses use different routes
Molecules interact without intent. Tasks can overlap and happen in different cell locations. Entry may deliver a genome without bringing the whole particle inside. Some infections stall or persist instead of immediately producing particles. Some viruses bud through membranes; others leave when a cell breaks open. The following slide traces influenza A as one specific example.
The checked sequence
Attachment, entry/uncoating, genome replication and protein synthesis, assembly, and release form a broad map. Mechanisms, locations and maturation steps vary. Sources: Medical Microbiology, Multiplication — https://www.ncbi.nlm.nih.gov/books/NBK8181/ ; Replication of Viruses — https://pmc.ncbi.nlm.nih.gov/articles/PMC7173441/
Influenza A follows its own route.
Influenza A is an enveloped RNA virus. Its RNA work uses the nucleus; cell ribosomes make proteins and particles bud at the membrane.
Influenza A example: RNA–protein complexes reach the nucleus; proteins are made outside it, and new particles bud at the plasma membrane. Icons are not to scale.
Notes and sources for this step
The example behind the map
Influenza A’s attachment protein binds suitable molecules containing sialic acid on the cell surface. The particle is taken into a membrane compartment; acidification enables membrane fusion and uncoating. Viral genome–protein complexes reach the nucleus, where viral enzymes transcribe and replicate RNA. Proteins made by cellular ribosomes and new genome complexes ultimately contribute to particles budding at the plasma membrane. Source: Swiss Institute of Bioinformatics, ViralZone: Orthomyxoviridae — https://viralzone.expasy.org/223
Do not turn one example into a universal rule
The nucleus is the cell compartment that holds its chromosomes. Influenza A shows why neither ‘all RNA viruses copy outside the nucleus’ nor ‘every virus bursts the cell to leave’ is a safe rule. Other viruses use different replication locations and release mechanisms. Sources: ViralZone: Orthomyxoviridae — https://viralzone.expasy.org/223 ; Medical Microbiology, Multiplication — https://www.ncbi.nlm.nih.gov/books/NBK8181/
A virus is not a tiny bacterium.
Bacteria are cells; viruses depend on cells to reproduce. Antibiotics treat certain bacterial infections, not viral infections.
Structural comparison, not to scale. Some bacteria also depend on host cells; dependence alone is not the definition of a virus.
Notes and sources for this step
Why the distinction matters
Bacteria are single-celled organisms. Viruses are different infectious agents with different reproduction mechanisms. Some bacteria also depend on host cells, so dependence alone is not the whole definition of a virus. Source: National Human Genome Research Institute, Bacteria — https://www.genome.gov/genetics-glossary/Bacteria ; Medical Microbiology, Multiplication — https://www.ncbi.nlm.nih.gov/books/NBK8181/
Antibiotics and the exception that is not an exception
Antibiotics do not work against viruses. A person with a viral illness may also develop a bacterial infection that separately needs antibiotics; this does not mean the antibiotic treats the virus. Some viral infections have specific antiviral treatments. Sources: CDC, Healthy Habits: Antibiotic Do’s and Don’ts — https://www.cdc.gov/antibiotic-use/about/ ; CDC, Manage Common Cold — https://www.cdc.gov/common-cold/treatment/
Which output depends on the blocked step?
Imagine a failure after genome delivery. Use the cell’s division of work to predict which downstream output changes.
A virus has delivered its genome, but cell ribosomes cannot read its RNA messages. Which output is directly interrupted?
- Production of new viral proteins needed for particles.
- Attachment of incoming particles to another cell’s surface.
- Delivery of the genome that has already entered this cell.
Read the answer
Ribosomes build proteins from RNA messages. Genome entry and genome copying alone cannot supply the new viral proteins needed for a generation.
Notes and sources for this step
The dependency to remember
This is an imagined blocked step, not a proposed treatment. A particle can bring proteins and enzymes, but producing a new generation requires protein synthesis using the host cell’s ribosomes. Source: Molecular Biology of the Cell, Cell Biology of Infection — https://www.ncbi.nlm.nih.gov/books/NBK26833/
Think: instructions, access, cell work, new particles.
Trace instructions, cell access, component production and release. Apply the questions to another virus before assuming it behaves like influenza.
Use this map as questions for another virus. Shared dependence does not imply a shared entry route, copying location or release mechanism.
Notes and sources for this step
Use the model on the next virus you meet
Ask these four questions before assuming that another virus behaves like influenza A. The host range, genome, entry route, copying location, and release mechanism may differ. The shared idea is dependence on a suitable cell, not one universal physical pathway. Sources: National Human Genome Research Institute, Virus — https://www.genome.gov/genetics-glossary/Virus ; Medical Microbiology, Multiplication — https://www.ncbi.nlm.nih.gov/books/NBK8181/