What Is a Virus? (The Biology)

Viruses are among the smallest and strangest things in biology. They cause some of humanity's most familiar illnesses, from the common cold to influenza, yet they are not even cells, and many scientists are not sure whether to call them alive at all. Understanding what a virus actually is, at the level of biology rather than headlines, helps make sense of why they behave so differently from other germs.

What a virus is

A virus is a microscopic infectious agent made of genetic material enclosed in a protein coat, which can only reproduce by entering a living cell and hijacking its machinery. On their own, viruses are inert. They cannot eat, grow, move under their own power or make copies of themselves. They are, in effect, a set of instructions in search of a cell to run them.

Viruses are extraordinarily small, far smaller than bacteria and most cells. While you could line up a few thousand average bacteria across a millimetre, you could fit many times more viruses in the same space. They are too small to see with an ordinary light microscope; their detailed structure was only revealed by the electron microscope.

What viruses are made of

Despite their reputation, viruses are structurally simple. A typical virus particle has just a few parts:

• Genetic material. At the core is a short set of genetic instructions, made of either DNA or its close relative RNA. This carries everything needed to make new copies of the virus once inside a host cell. The chemistry of these molecules is the same one explained in our guide to what DNA is.
• A protein coat (capsid). Surrounding and protecting the genetic material is a shell built from protein. Its often geometric, highly symmetrical shape is one of the hallmarks of viruses.
• An envelope (in some viruses). Many viruses, including the influenza and coronavirus families, also have an outer fatty membrane studded with proteins. These surface proteins help the virus attach to and enter host cells, and are often what soap and disinfectants disrupt.

That is essentially it. A virus has no cell wall, no nucleus, no internal machinery for making energy or proteins. It carries the blueprint but none of the factory.

How viruses reproduce

Because a virus lacks the equipment to copy itself, it must borrow someone else's. This is the heart of how viruses work, and it follows a general pattern:

1. Attachment. The virus latches onto a specific host cell, recognising particular molecules on the cell's surface. This is why many viruses infect only certain species or certain types of cell.
2. Entry. The virus, or just its genetic material, gets inside the cell.
3. Takeover. The viral genes hijack the cell's own machinery, redirecting it to read the viral instructions instead of the cell's.
4. Assembly. The cell churns out the components of new virus particles, which are assembled into copies.
5. Release. The new viruses leave, often bursting and destroying the host cell, and go on to infect others.

A single infected cell can produce many thousands of new virus particles. This explains how an infection can spread so rapidly through the body, and why symptoms can escalate quickly.

Are viruses alive?

This is one of biology's genuinely open questions, and the answer depends on how you define life. Viruses share some features with living things and lack others.

In favour of "alive": they contain genetic material, they can mutate and evolve over time through natural selection, and they reproduce, after a fashion.

Against "alive": they are not made of cells, they cannot grow, they generate no energy of their own, and they cannot reproduce independently. Outside a host, a virus is essentially an inert chemical package.

For these reasons, many scientists describe viruses as existing on the boundary between living and non-living, sometimes calling them "at the edge of life". It is less a failure of biology to decide than a sign that life does not come with sharp, tidy edges.

Viruses versus bacteria

Viruses and bacteria are often lumped together as "germs", but biologically they could hardly be more different.

A bacterium is a complete, single-celled living organism. It can take in nutrients, generate energy and reproduce entirely on its own. Many bacteria are harmless or even helpful, such as those in the human gut. A virus, by contrast, is not a cell and is wholly dependent on a host to multiply.

This distinction has a vital practical consequence: antibiotics work against bacteria, not viruses. Taking antibiotics for a viral illness such as a cold or the flu does no good and contributes to the serious problem of antibiotic resistance. The body's own defences against viruses, and the way medicine can prime them in advance, are explained in our guide to how vaccines work.

Why viruses matter

Viruses are a huge part of the living world, and not only as causes of disease. They are the most numerous biological entities on Earth, found everywhere from the oceans to the soil to inside our own bodies. They shape ecosystems, influence the evolution of other organisms, and are even used as tools in medicine and biotechnology.

Of course, their effect on human health is what most people notice. By invading and damaging cells, and by triggering the immune system's response, viruses cause illness. This is general scientific information about the biology of viruses, not medical advice; for guidance on any specific infection or symptoms, consult the NHS or a healthcare professional, and call NHS 111 if you need urgent help.

The bottom line

A virus is a tiny package of genetic material wrapped in a protein coat, far smaller than a cell, that cannot reproduce on its own. To multiply, it must invade a living cell and hijack that cell's machinery to make copies of itself. Because it lacks the features of a true cell, a virus sits on the blurred boundary between living and non-living, and it is fundamentally different from a bacterium, which is why antibiotics do not touch it. Simple in structure but profound in impact, the virus is one of biology's most fascinating puzzles.