Is a Virus Alive? Tested Against All Seven Criteria

Seven characteristics, taken one at a time

School biology defines a living thing by what it does, usually with the mnemonic MRS GREN. Rather than arguing about definitions in the abstract, put a virus particle through the list and record the verdict each time.

Read down the column of verdicts and the answer is not close. On its own, outside a host, a virus particle does none of the seven things. It is chemically inert. Preparations of some viruses can be crystallised and stored on a shelf, which is not a thing any organism does.

• Movement — fails. A virus has no means of moving itself; it drifts, is breathed in, or is carried in a droplet.
• Respiration — fails. It has no metabolism at all and releases no energy from anything.
• Sensitivity — fails. Binding to a receptor on a cell is chemistry taking place on its surface, not a response to a stimulus.
• Growth — fails. A virus particle is assembled at full size and never gets any bigger.
• Reproduction — fails alone, succeeds only inside a host. This is the decisive one.
• Excretion — fails. There is no waste, because there is no metabolism to produce any.
• Nutrition — fails. It takes nothing in and requires no food.

What it does have, which is not nothing

A virus is a length of genetic material — DNA or RNA, single or double stranded — inside a protein coat called a capsid, sometimes wrapped in a lipid envelope stolen from the membrane of the last cell it left. That is the whole organism, if organism is even the word.

It carries genes. Those genes mutate. Variants that copy themselves more successfully become more common, and variants that do not, disappear. That is natural selection operating on a population, and it is not a metaphor: it is the reason influenza vaccines are reformulated for each season and the reason antiviral resistance emerges. Heredity plus variation plus differential survival is the engine that drives all of life, and a virus has all three.

So the honest position is that a virus has the information half of life and none of the machinery half. It is the instructions without the factory.

The sharpest line in the whole question

If a student needs one fact to hang the argument on, it is this: a virus has no ribosomes.

Every cell that has ever been found — bacterium, archaean, plant, animal, fungus — carries ribosomes and makes its own proteins. It is the one piece of equipment shared by every branch of the tree of life. A virus has none, cannot make a single protein of its own, and must have the host's ribosomes read its genes and build its coat for it.

That is the whole of the dependency, stated precisely. Not merely that a virus needs a cell, but exactly what it needs from that cell and why it cannot supply it. It is far more useful in an answer than saying a virus needs a host to reproduce, because it names the missing part.

Why biologists have not simply closed the case

The tidy answer above was much tidier before the giant viruses. Mimivirus was isolated from water in a cooling tower in Bradford, England, and was mistaken for a bacterium for years until electron microscopy showed what it was. It carries a double-stranded DNA genome running past a million base pairs, with roughly a thousand protein-coding genes — larger than the genomes of some bacteria — and it includes genes related to protein synthesis that no ordinary virus has any business owning.

Mimivirus can itself be infected by smaller viruses, called virophages, and appears to carry a defence system against them. A parasite with its own parasites and its own immune arrangements is not easy to file under inert particle. Some researchers have argued that giant viruses represent a distinct branch of the tree of life; others hold that they acquired those genes from their hosts over time. The disagreement is live and it is about where a boundary should be drawn, not about what a virus does.

There is also a fair case that the question is malformed. Alive is a word coined for things like us, and applying it to a replicator that borrows a factory may simply be asking a category to do work it was never built for. Some biologists prefer to describe viruses accurately — obligate intracellular parasites that evolve — and let the label go.

How to answer it in an exam without hedging

Marks here are given for criteria, not for philosophy, and a student who writes a balanced essay about the nature of life while naming nothing specific will score badly.

State the position: viruses are not classified as living organisms. Then justify it with named characteristics they fail — no metabolism or respiration, no growth, no independent reproduction. Then concede accurately: they contain genetic material and they evolve. If there is room, name the missing ribosomes.

In our Key Stage 3 and GCSE biology sessions we treat this as the model answer for every is it alive question, because the structure — state, justify with named criteria, concede — is exactly what the mark scheme rewards, and it works just as well for seeds, spores and prions.