Viruses — How They Hijack Cells

A virus occupies a strange place in biology. Most scientists don't classify viruses as "alive" in the same sense as cells, because they cannot reproduce, generate energy, or carry out metabolism on their own. Outside a cell, a virion (a complete virus particle) is essentially inert — genetic material wrapped in a protective coat, waiting for a host. Every virus is built from a small set of parts: - Genome — the genetic material, either DNA or RNA (unlike cells, which always use DNA), single- or double-stranded. - Capsid — a protein shell that protects the genome and gives the virus its shape. - Envelope (in some viruses) — an outer membrane stolen from the host cell as the virus exits, studded with viral proteins. Enveloped viruses (influenza, HIV, coronaviruses) are generally more fragile outside the body, which is why alcohol-based hand sanitiser — which dissolves lipid membranes — is effective against them. - Surface proteins — proteins on the capsid or envelope that let the virus recognise and attach to specific host cells. SARS-CoV-2's "spike protein" is the best-known example. Viruses are typically around 1,000 times smaller than a bacterium — far too small to see with a standard light microscope. Because a virus has no metabolism of its own, it depends entirely on hijacking a living cell's machinery to reproduce. That single fact explains almost everything about how viruses cause disease.

Different viruses vary in detail, but almost all follow the same basic cycle to reproduce inside a host cell. 1. Attachment — the virus's surface proteins bind a specific receptor on the host cell. This is why viruses tend to infect specific cell types: a virus can only enter cells displaying the receptor it recognises. SARS-CoV-2's spike protein binds the ACE2 receptor, found on cells lining the lungs and airways — which is why it primarily causes respiratory disease. 2. Entry — the virus (or just its genome) gets into the cell, either engulfed by endocytosis or by fusing its envelope directly with the cell membrane. 3. Uncoating — the protein capsid is removed, releasing the viral genome into the cytoplasm. 4. Replication and synthesis — the hijacking step. The virus commandeers the host cell's ribosomes, enzymes, and raw materials to make copies of its genome and viral proteins. Some viruses, including retroviruses like HIV, carry an enzyme called reverse transcriptase, which converts their RNA genome into DNA so it can be spliced into the host's own chromosome — a permanent infection of that cell's DNA. 5. Assembly — new genomes and proteins self-assemble into thousands of fresh virus particles. 6. Release — new particles exit to infect other cells, either by lysis (bursting the cell, killing it) or by budding through the membrane (a gentler exit for enveloped viruses that can keep the host cell alive and producing virus for longer). The entire cycle can take as little as a few hours, which explains how quickly an infection can spread through the body — and between people.

Symptoms come from a combination of direct cell damage and the immune system's response to infection — and often it is the immune response itself that causes much of the discomfort. Direct cytopathic effects: - Cell lysis — many viruses kill the cells they infect, either bursting them on release or disrupting essential functions. Killing enough cells in a tissue, like the airway lining, directly causes symptoms. - Functional disruption — a virus can cripple a cell's job without killing it outright, as HIV does to helper T cells, gradually weakening the body's defences. The immune response — a double-edged sword: fever, fatigue, and inflammation during a viral infection are largely caused by the immune system itself — the release of signalling molecules called cytokines that coordinate the attack on infected cells. This response is usually protective, recognising and destroying infected cells before the virus can spread further. But an excessive response can itself cause serious harm: a cytokine storm — an overwhelming, poorly controlled release of inflammatory signals — can damage healthy tissue and drives severe illness in some viral infections. Latency — viruses that hide: some viruses insert their genome into the host cell's DNA (or persist quietly within it) without actively replicating, hiding from the immune system for long periods. Herpesviruses (cold sores, chickenpox/shingles) and HIV are classic examples. The virus can later reactivate, causing disease again — sometimes years after the original infection.

Treating viral infections is harder than treating bacterial ones, because viruses use the host cell's own machinery — a drug that disrupts viral replication risks disrupting normal cell function too. Even so, several strategies successfully target unique steps in the viral cycle. Blocking entry — some drugs block the interaction between a viral surface protein and the host receptor, stopping the virus getting in at all. Maraviroc, used in HIV treatment, blocks a co-receptor needed for HIV entry into immune cells. Blocking replication enzymes — many successful antivirals target enzymes the virus brings with it, which human cells don't have (or have a different version of), minimising side effects: - Nucleoside analogues (e.g. aciclovir for herpes) resemble the building blocks of DNA/RNA. The viral polymerase mistakenly incorporates them into the new genome, but they block further chain extension — halting replication. - Reverse transcriptase inhibitors block the enzyme HIV uses to convert its RNA into DNA — a step human cells never perform, making it an excellent drug target. - Protease inhibitors block an enzyme some viruses (including HIV) need to cut large viral protein chains into functional pieces during assembly. Blocking release — neuraminidase inhibitors (such as oseltamivir, used for influenza) block the enzyme a virus needs to release new particles from infected cells, slowing the spread of infection through the body. This is also why antibiotics are useless against viral infections like the common cold or flu: antibiotics target structures specific to bacteria — cell walls, bacterial ribosomes, bacterial DNA-copying enzymes — none of which a virus has, because a virus isn't a cell. Using them anyway contributes to antibiotic resistance without treating the actual infection.

If antiviral drugs are difficult because viruses hide inside our own cells, the best defence is often to prevent infection altogether — or equip the immune system to respond overwhelmingly fast before the virus can establish itself. This is the principle behind vaccination. A vaccine exposes the immune system to a harmless version or component of a pathogen, allowing it to build memory cells without the person experiencing the actual disease. If the real virus is encountered later, the immune system recognises it immediately and mounts a fast, powerful response — often preventing illness altogether, or making it much milder. | Vaccine type | How it works | Example | |---|---|---| | Live attenuated | Weakened virus still triggers immunity but can't cause significant disease | MMR (measles, mumps, rubella) | | Inactivated | Killed virus, can't replicate but still displays recognisable surface proteins | Some flu and polio vaccines | | Subunit | Only a key viral protein is delivered, not the whole virus | Hepatitis B vaccine | | mRNA | Instructions tell the person's own cells to make a harmless viral protein temporarily | COVID-19 vaccines | The mRNA approach, used prominently for COVID-19, can be developed and adapted far more quickly than traditional methods, since it requires only the genetic sequence of the target protein rather than growing or engineering the virus itself. Herd immunity: when enough of a population is immune — through vaccination or prior infection — a virus struggles to find new susceptible hosts and its spread slows dramatically, indirectly protecting people who cannot be vaccinated themselves, such as those with weakened immune systems. This population-level effect is one of the most powerful tools in public health.