A complete virus particle, a virion: an icosahedral protein capsid (purple) holding the genome (gold), wrapped in a lipid envelope (cyan) studded with glycoprotein spikes (orange). Press Play or step forward to watch it infect a host cell.
A virus is like a USB drive full of instructions but with no computer of its own to run them. The "instructions" are a tiny genome; the plastic case is a protein shell called a capsid. By itself the drive does nothing — it has no power and no machinery. Only when it is plugged into a working "computer," a living host cell, do the instructions run and start making copies. That is why a virus is called an obligate intracellular parasite: it absolutely needs a host cell to reproduce.
The minimum virus is just two parts: a genome of nucleic acid (DNA or RNA, never both) and a capsid built from repeating protein blocks called capsomeres, which is why capsids are so symmetric — often a $20$-sided icosahedron or a helical rod. Many animal viruses add an envelope: a stolen patch of lipid membrane carrying viral glycoprotein spikes. Those spikes are the key; the host cell's receptor is the matching lock. A rough size sense: most viruses are about $20$–$300$ nanometers across, roughly $100\times$ smaller than the bacteria they are often confused with.
Replication runs in order: attachment (spike binds receptor) → entry & uncoating (the virus gets in and the capsid releases the genome) → replication & gene expression (the host's ribosomes and enzymes copy the genome and build new viral proteins) → assembly (new genomes and capsids snap together) → release (new virions bud out or burst the cell). The cycle selector switches the outcome: the lytic cycle makes virus immediately and destroys the cell, while the lysogenic cycle instead tucks the viral genome into the host's own DNA as a dormant prophage that waits, sometimes for many cell generations, until a trigger flips it back to lytic.
Turn on Cutaway to see the gold genome coiled inside the capsid, then toggle the Envelope and Spikes off to compare an enveloped virus with a naked one. Press Play and freeze on the attachment step to see a single spike dock onto its matching receptor. Finally, switch the cycle from lytic to lysogenic and step to the end to watch the genome integrate into the host DNA instead of bursting the cell.
| Stage | What happens | Key players |
|---|---|---|
| 1 · Virion | A complete particle: genome inside a capsid, often wrapped in an envelope with spikes | Genome, capsid, envelope, spikes |
| 2 · Attachment | A surface spike binds a specific receptor on the host cell — a key-and-lock match | Spike, host receptor |
| 3 · Entry & uncoating | The virus enters (membrane fusion or endocytosis); the capsid comes apart and frees the genome | Envelope/capsid, host membrane |
| 4 · Replication & expression | The genome is copied many times and read to build new viral proteins, using host machinery | Host ribosomes, polymerases |
| 5 · Assembly | New genomes and freshly made capsomeres self-assemble into new virions | Capsid proteins, new genomes |
| 6 · Release | New virions bud through the membrane (gaining an envelope) or burst the cell (lysis) | Host membrane, new virions |
In the lytic cycle the virus runs straight through replication and release, typically destroying the host cell. In the lysogenic cycle, best known in bacteria-infecting viruses (bacteriophages) and in temperate animal viruses, the viral genome instead integrates into the host chromosome as a prophage (or provirus). It then replicates silently every time the host cell divides, so a single infection can be passed to thousands of descendant cells with no new virus made. A stressor — UV light, chemicals, or DNA damage — can trigger induction, excising the viral genome and switching it into the lytic cycle.
A viral genome may be DNA or RNA, single- or double-stranded. DNA viruses often borrow the host's own DNA polymerases in the nucleus; many RNA viruses must supply an RNA-dependent RNA polymerase because host cells do not normally copy RNA into RNA. Retroviruses such as HIV carry reverse transcriptase, copying their RNA into DNA that integrates into the host genome — a built-in lysogenic-style step. These differences are why antiviral drugs and vaccines must be tailored to each virus.
Every virus has two essential parts: a genome of nucleic acid (DNA or RNA, never both) carrying the instructions, and a protein coat called a capsid that protects it. The capsid is built from many copies of a few protein subunits (capsomeres), which is why so many capsids are symmetric — often icosahedral (a 20-sided ball) or helical (a rod). Some viruses add an envelope: a piece of lipid membrane taken from a previous host, studded with viral glycoprotein spikes. Naked viruses skip the envelope.
Key takeaway: genome plus capsid is the minimum; the envelope and spikes are extras many but not all viruses carry.Most biologists place viruses in a grey zone. On their own, a virus particle (virion) has no metabolism, makes no energy, cannot grow, and cannot copy itself — it behaves like a complex chemical package. But once inside a host cell it hijacks the cell's enzymes, ribosomes, and energy to make hundreds of copies, acting very much like something alive. Because they cannot reproduce without a host, viruses are called obligate intracellular parasites.
Key takeaway: a virus is inert outside a cell and only shows life-like activity by borrowing a living cell's machinery.Both replicate the virus but differ in timing. In the lytic cycle the virus immediately takes over, makes many new virions, and bursts (lyses) or buds out, usually killing the cell. In the lysogenic cycle the viral genome instead inserts into the host's own DNA as a quiet prophage, copied along with the host genome each time the cell divides, often causing no harm. A trigger such as stress or UV damage can later switch it into the lytic cycle (induction).
Key takeaway: the lytic cycle destroys the cell right away, while the lysogenic cycle hides in the host genome and waits.The envelope is a lipid bilayer wrapped around the capsid, taken from the membrane of the cell the virus last left, with viral glycoprotein spikes embedded in it that bind the next host cell like a key fitting a lock. Because the envelope is made of fat, soap and alcohol sanitizers tear it apart: soap dissolves the lipid membrane, and without its envelope an enveloped virus such as influenza or a coronavirus can no longer attach to or enter cells. Naked viruses, lacking a fatty envelope, are generally tougher.
Key takeaway: the envelope carries the spikes the virus needs to get in, and because it is lipid, soap breaks it and disables the virus.A virus cannot force its way in; it must be let in. First comes attachment: the virus's surface proteins (spikes, or capsid proteins on naked viruses) bind specific receptor molecules on the host. This is highly specific, like one key fitting one lock, which is why a virus only infects certain species and cell types. It then enters by fusing its envelope with the membrane and releasing the capsid, or by being swallowed in a bubble (endocytosis). The capsid then uncoats, freeing the genome.
Key takeaway: infection starts with a specific spike-to-receptor match, then the virus enters and uncoats to release its genome.No. A genome can be DNA or RNA, single- or double-stranded, and that sets the copying strategy. DNA viruses often use the host's own DNA-copying enzymes in the nucleus. Many RNA viruses bring or build an RNA-dependent RNA polymerase, since host cells do not normally copy RNA into RNA. Retroviruses such as HIV carry reverse transcriptase, copying their RNA into DNA that integrates into the host chromosome. These differences are why antivirals must be tailored to specific viruses.
Key takeaway: the genome type sets the copying strategy, which is why viruses replicate in several distinct ways.Antibiotics attack structures found in bacteria — the bacterial cell wall, bacterial ribosomes, or bacterial enzymes. Viruses have none of these: no cell wall, no ribosomes, no metabolism of their own, since they borrow the host cell's machinery. With nothing for an antibiotic to target, antibiotics simply do not affect viruses, which is why they are useless against colds, flu, and COVID-19. Viral infections need antiviral drugs that block specific viral steps, or vaccines that prime the immune system.
Key takeaway: antibiotics target bacterial machinery that viruses lack, so viral infections need antivirals or vaccines instead.