Viruses reproduce by taking over a living cell, because they can’t copy themselves on their own. A virus attaches to a cell, gets its genetic material inside, uses the cell’s machinery to make viral parts, assembles new virus particles and releases them to infect more cells.
Why a virus needs a host cell at all
A virus is little more than a set of instructions in a protective wrapper. The National Human Genome Research Institute describes it as a segment of DNA or RNA surrounded by a protein coat, and notes that a virus cannot replicate alone. It has no way to make proteins or energy of its own.
That is the key difference from bacteria, which are cells that divide by themselves. Outside a host, a virus particle, called a virion, is inert. Inside the right cell, its genes redirect the cell’s own equipment to build copies of the virus.
The five stages of viral replication
Textbooks split the cycle up in slightly different ways, but the sequence is the same.
| Stage | What happens |
|---|---|
| 1. Attachment | Proteins on the virus bind to specific receptor molecules on the cell surface. |
| 2. Entry and uncoating | The virus or its genome gets inside, by fusing with the membrane or being swallowed by the cell, and the protein coat breaks down. |
| 3. Replication | The cell’s machinery copies the viral genome and makes viral proteins. |
| 4. Assembly | New genomes and coat proteins come together into new virions. |
| 5. Release | New viruses leave the cell, either by bursting it open or by budding out through the membrane. |
The final stage decides what happens to the cell. Viruses released by bursting the cell open destroy it on the way out. Viruses that bud through the membrane can leave without killing the cell right away, wrapping themselves in a piece of that membrane as they go, which is where enveloped viruses get their outer layer.
For a stretch after entry, often called the eclipse phase, no complete virus particles can be found at all. The virus exists only as instructions being carried out.
Receptors decide which cells a virus can infect
Attachment works like a key fitting a lock. A virus’s surface proteins only fit certain receptors, and a cell without the matching receptor is usually safe from that virus. This is a big part of why many viruses infect only certain species, and why some target particular tissues such as the airways or the gut.
Enveloped viruses carry a fatty outer layer taken from a previous host cell, studded with their own proteins. Viruses without an envelope rely on the protein shell itself to make contact.
How DNA and RNA viruses copy their genes differently
DNA viruses generally use the host’s own systems to copy their DNA and make messenger RNA, and many do this in the cell nucleus. RNA viruses use their RNA genome as a template instead, usually with an enzyme they bring along or make, since cells don’t normally copy RNA from RNA.
Copying isn’t perfect. Small errors creep into new viral genomes, and when a change happens to help a virus spread or slip past the immune system, those copies can become more common over time. That is one reason some viruses change from season to season.
Retroviruses such as HIV take a detour. They use an enzyme called reverse transcriptase to turn their RNA into DNA, and that DNA is inserted into the host’s own chromosomes, where the cell reads it like one of its own genes.
Lytic and lysogenic cycles in bacteriophages
Viruses that infect bacteria, known as bacteriophages, show two strategies clearly. In the lytic cycle, the phage takes over the cell, makes new phages and destroys the cell as they burst out. In the lysogenic cycle, the phage genome joins the bacterial chromosome and is copied quietly every time the bacterium divides. That hidden viral DNA is called a prophage, and it can later switch into the lytic cycle.
Some animal viruses behave in a comparable way, lingering in the body in a dormant, latent state. Questions about a specific infection, symptoms or treatment belong with a doctor.
Frequently asked questions
Are viruses alive?
Scientists still debate it. Viruses carry genes and evolve, yet they cannot grow, make energy or reproduce without a host cell. Many biologists describe them as sitting at the edge of life, active only when they are inside a cell they can use.
How fast can a virus make copies of itself?
It varies widely by virus and by cell type. Some complete a cycle within hours, while others take far longer or stay dormant for years. A single infected cell can release many new virus particles, which is why infections can spread quickly through tissue.
Can viruses reproduce on surfaces like doorknobs?
No. A virus on a surface cannot make more copies, since reproduction requires a living host cell. Some viruses can remain infectious on surfaces for a while, which is why handwashing and cleaning help, but the numbers on a surface only fall over time.
Do viruses have cells?
No. Viruses are acellular, meaning they are not made of cells at all. A virus particle is a strand of DNA or RNA packed inside a protein coat called a capsid, and some also carry a fatty envelope taken from a host cell's membrane. Lacking a cell's machinery, a virus must infect a living cell to reproduce.
Do antibiotics work on viruses?
No. Antibiotics work by killing bacteria or making it hard for them to grow and multiply, and viruses are not bacteria. A virus copies itself inside your own cells using their machinery, so antibiotics can't stop it. The CDC notes that antibiotics do not treat viral illnesses such as colds and flu.
