A virus is a tiny infectious agent that can reproduce only by using the machinery of a living cell. Unlike bacteria, fungi, plants, and animals, viruses are not made of cells. They do not have their own ribosomes, energy-producing systems, or other cellular machinery needed to carry out independent life processes.
A virus is essentially genetic information packaged for delivery into a host cell. Its genetic material may be DNA or RNA and is enclosed in a protective protein shell called a capsid. Some viruses also have an outer membrane-like envelope taken from a host cell and modified with viral proteins.
Viruses are extraordinarily diverse. Some infect humans and cause illnesses such as influenza, measles, or COVID-19; others infect animals, plants, fungi, or bacteria. Despite their differences, viruses share a central strategy: they enter suitable host cells, redirect cellular processes toward making viral components, assemble new virus particles, and release them so the infection can spread.
Viruses are not cells
The distinction between a virus and a cell is fundamental.
A typical cell contains genetic material along with molecular machinery that reads that information, produces proteins, obtains and uses energy, maintains internal conditions, and reproduces. A virus contains genetic information and the structures needed to protect and deliver it, but it lacks the machinery required to perform these activities independently.
For this reason, viruses cannot normally reproduce in a nutrient-rich environment by themselves. A virus outside a host cell is generally an inactive infectious particle, often called a virion. It may remain stable for some period of time, but it is not carrying out metabolism or producing new virus particles.
Once inside an appropriate living cell, however, viral genetic material can take control of cellular machinery. The infected cell becomes, in effect, a production site for new viral components.
Whether viruses should be considered “alive” is therefore a matter of how life is defined. Viruses have genetic information and evolve through natural selection, but they lack independent cellular metabolism and reproduction.
What is a virus made of?
Although viral structures vary widely, most viruses have two fundamental components: genetic material and a protein capsid.
The genetic material contains the instructions needed to produce new virus particles. Depending on the virus, those instructions are encoded in DNA or RNA. Viral genomes can differ greatly in size and organization, and they may consist of a single molecule or multiple segments.
The capsid surrounds and protects the genome. It is built from viral proteins called capsomeres, which assemble into an organized structure. The shape of the capsid differs among viruses and can be an important feature of viral classification.
Some viruses have an additional envelope surrounding the capsid. The envelope is a lipid membrane derived from a host cell membrane or an internal membrane of the host cell. Embedded in it are viral proteins, often called spike proteins, that can help the virus recognize and enter susceptible cells.
The envelope also creates an important vulnerability: enveloped viruses are generally more sensitive to substances and conditions that disrupt lipid membranes than non-enveloped viruses.
How a virus recognizes a cell
A virus cannot infect every cell it encounters. Infection depends partly on whether the cell has the right molecular structures for the virus to attach to and enter.
These structures are usually proteins or other molecules on the surface of the cell. Viral proteins recognize particular features of them, much like a molecular interaction between two complementary structures. This recognition helps determine a virus’s host range—the types of organisms, tissues, or cells it can infect.
This specificity also helps explain why different viruses tend to cause different diseases. A virus that can efficiently enter and reproduce in cells of the respiratory tract may cause a respiratory infection, while another virus may preferentially infect cells in the liver, nervous system, skin, or other tissues.
The presence of a suitable receptor, however, is not always enough. After attachment, the virus must also be able to enter the cell, release its genome, reproduce successfully using the cell’s machinery, and overcome or evade cellular defenses.
The main stages of viral infection
Viral infections vary considerably, but the basic process can be understood as a sequence of steps: attachment, entry, uncoating, replication and gene expression, assembly, and release.
1. Attachment
The infection begins when viral molecules bind to specific molecules on the surface of a susceptible cell.
This attachment is not simply a matter of physical contact. Viral proteins must interact with compatible molecules on the host cell. These interactions help determine which cells a particular virus can infect.
Attachment can also trigger changes that help the virus enter the cell.
2. Entry into the cell
After attachment, the virus or its genetic material must cross the cell’s protective membrane.
Different viruses accomplish this in different ways. Some enveloped viruses can fuse their envelope with the host cell membrane, allowing the viral contents to enter. Others are taken into the cell inside membrane-bound compartments through a process known as endocytosis.
Non-enveloped viruses use other mechanisms to cross the membrane or deliver their genome into the cell.
The details matter because the virus must get its genome into the correct cellular environment without destroying it before replication can begin.
3. Uncoating
Once inside, many viruses must remove their protective capsid or otherwise expose their genetic material. This step is called uncoating.
The viral genome then becomes available to the host cell’s molecular machinery or to viral proteins that have entered the cell with it.
At this point, the infection shifts from delivery to production.
4. Viral gene expression and genome replication
The virus must make the proteins and genetic material needed to produce new virions.
How this happens depends heavily on whether the virus carries DNA or RNA and on the particular type of genome it has.
Some viral genomes can be read relatively directly by the host cell’s protein-making machinery. Others require specialized viral enzymes to copy or convert their genetic information before useful viral proteins can be produced.
For example, many RNA viruses carry or produce enzymes that copy RNA from an RNA template. Retroviruses use an enzyme called reverse transcriptase to make a DNA copy from their RNA genome; that DNA can then become part of the infected cell’s genetic environment.
Viruses also redirect cellular resources toward their own reproduction. Viral proteins can alter cellular gene expression, membrane production, metabolism, and other processes. The result is a cell whose resources are increasingly devoted to producing viral material.
5. Assembly
New viral genomes and proteins are assembled into complete virus particles.
Assembly is not necessarily a simple process of putting components together randomly. Viral proteins and genomes often contain molecular signals that guide them toward the correct locations and interactions.
For some viruses, assembly occurs in the cell’s cytoplasm. Others assemble partly or entirely in the nucleus or at particular cellular membranes.
Enveloped viruses acquire their lipid envelope as new particles bud through a suitable host-cell membrane containing viral proteins.
6. Release
New virions must leave the infected cell to reach other cells.
Some viruses cause the host cell to break apart, or lyse, releasing many virus particles at once. Other viruses, particularly many enveloped viruses, leave gradually by budding from a cellular membrane. The cell may survive for some time while continuing to produce and release virus particles, although infection can eventually damage or kill it.
The newly released virions can then encounter other susceptible cells and begin the cycle again.
How viruses take over cellular machinery
A virus does not usually carry everything required to manufacture a new virus particle. Instead, it supplies genetic instructions that redirect machinery already present in the host cell.
One crucial resource is the ribosome, the molecular machine that builds proteins. Viral messenger RNA, when compatible with the host’s translation machinery, can be used to produce viral proteins just as cellular messenger RNA is used to produce cellular proteins.
Viruses also depend on host molecules for energy, raw materials, membranes, and other cellular functions. Some viruses encode enzymes that the host cell lacks or that are especially useful for copying the viral genome.
This dependence on the host is one reason viruses are difficult to classify simply as organisms or nonliving chemicals. They are genetically active and capable of evolution, but their reproductive cycle is inseparable from living cells.
What happens to the infected cell?
Viral infection can have very different effects on a cell.
In some infections, viral replication severely disrupts normal cellular functions and kills the cell. This can contribute directly to tissue damage and disease. In other cases, infected cells remain alive while continuously producing virus.
Some viruses establish persistent infections, in which viral genetic material or virus production remains in the body for long periods. Others can enter a latent state in which the viral genome remains in cells with little or no production of new infectious particles for a period of time. Latency is particularly characteristic of certain viruses, including herpesviruses.
Viral infection can also alter the behavior of a cell without immediately killing it. Depending on the virus and host, infection may change cell growth, immune signaling, or gene activity.
How the immune system detects viruses
The body has several layers of defense against viral infection.
The first barriers include physical and chemical defenses such as skin, mucous membranes, and substances that make some tissues hostile to invading microbes. If a virus enters susceptible cells, the innate immune system can detect signs of infection and initiate an immediate response.
Infected cells can produce interferons, signaling proteins that help neighboring cells increase their antiviral defenses and alert other parts of the immune system. Other immune cells can recognize and destroy infected cells.
The adaptive immune system provides more targeted defenses. Antibodies can bind to viruses or viral proteins outside cells and, in some cases, prevent them from attaching to or entering cells. T cells can recognize infected cells and help coordinate immune responses or kill infected cells.
The immune response is a major reason many viral infections eventually stop. It can also contribute to symptoms and tissue damage, because inflammation and the destruction of infected cells are part of the body’s defense.
Why some viruses change over time
Viral genomes can accumulate genetic changes through mutation and, for some viruses, through processes such as recombination or reassortment. Natural selection then favors variants that reproduce more successfully under particular conditions.
The rate and nature of viral evolution differ among viruses. RNA viruses often have substantial genetic variation because their genome-copying mechanisms can be relatively error-prone, although some RNA viruses have proofreading mechanisms. Viruses with segmented genomes can sometimes exchange genome segments when two related viruses infect the same cell, a process known as reassortment.
Evolution does not mean that every mutation makes a virus more harmful. A genetic change can have little effect, reduce viral fitness, or alter characteristics such as transmissibility, immune recognition, or the ability to reproduce in particular cells.
Why antibiotics do not treat viral infections
Antibiotics work against bacteria by targeting bacterial structures or processes. Viruses do not have those bacterial structures or independent cellular processes, so antibiotics generally have no direct effect on viral replication.
Some viral infections can be treated with antiviral drugs. These medications work by interfering with specific stages of a viral life cycle. Depending on the drug and virus, a treatment might inhibit genome copying, prevent viral proteins from functioning, block processing of viral components, or interfere with the release of new virions.
Vaccines work differently. Rather than directly eliminating a virus after infection, vaccination prepares the immune system to recognize particular viral components or infected cells more effectively if exposure occurs.
Viruses, bacteria, and other microbes are different
The word “germ” can make very different biological entities sound interchangeable. They are not.
Bacteria are single-celled organisms with their own cellular machinery. They can reproduce independently under suitable conditions.
Fungi are organisms whose cells contain nuclei and other complex structures. Some fungi are single-celled, such as yeasts, while others form multicellular structures.
Protozoa are diverse single-celled eukaryotic organisms. Some species can cause human disease.
Viruses, by contrast, are acellular infectious agents that depend on host cells for reproduction.
This distinction matters medically. A bacterial infection and a viral infection may produce similar symptoms, but the underlying biology and appropriate treatments can be completely different.
The essential idea
A virus is a package of genetic information specialized for entering a susceptible cell and using that cell to make more virus. Its capsid—and, in some viruses, its envelope—protects the genome and helps deliver it to the right cellular environment.
Once inside, the virus releases its genetic instructions, uses or supplies the machinery needed to copy them and produce viral proteins, assembles new particles, and releases those particles to infect additional cells.
That basic strategy can produce an enormous range of biological effects, from brief infections that resolve without lasting harm to persistent infections that alter cells for years. Understanding the viral life cycle explains not only what viruses are, but also why they cause disease, how the immune system fights them, and why antiviral medicines and vaccines can target viral infections in fundamentally different ways.
