Viruses sit on the border between living and non-living. They are not made of cells, they cannot do anything on their own — yet inside a living host they hijack the machinery of life and multiply.
1 · Living or non-living?
A virus is an acellular particle — it has no cell, no cytoplasm, no organelles, and no metabolism of its own. Outside a host it is completely inert (some can even be crystallised, like a chemical). Yet inside a living cell it reproduces, carries genetic material, and evolves. So viruses are usually described as being on the borderline of life.
2 · Structure
A virus particle (a virion) is extremely small (about 20–300 nm — visible only with an electron microscope) and very simple:
A core of nucleic acid — either DNA or RNA (never both) — carrying the genes.
A protein coat, the capsid, built from repeating units called capsomeres, which protects the nucleic acid.
Some viruses also have an outer envelope of lipid (e.g. influenza, HIV).
A bacteriophage (a virus that infects bacteria) has a more complex shape: a head (capsid + DNA), a tail, and tail fibres for attaching to the host.
3 · Viruses are obligate parasites
Because a virus has no machinery of its own, it can only multiply inside a living host cell — it is an obligate intracellular parasite. Each kind of virus is usually specific to a particular host or cell type.
4 · How a virus multiplies — the lytic cycle
Attachment — the virus binds to specific receptors on the host cell surface.
Penetration — it injects (or releases) its nucleic acid into the cell.
Biosynthesis — the viral genes take over, making the cell build new viral nucleic acid and proteins.
Assembly — new virus particles are put together.
Release (lysis) — the cell bursts, freeing many new viruses to infect more cells.
In an alternative lysogenic cycle, the viral DNA inserts into the host's DNA and stays dormant, copied each time the cell divides, until something triggers it back into the lytic cycle.
5 · Importance
Viruses cause many diseases — influenza, measles, polio, hepatitis, dengue, HIV/AIDS and COVID-19 in humans; and diseases of plants and animals (e.g. tobacco mosaic virus). They also matter in a useful way: in vaccines, in gene therapy and as tools in biotechnology.
Why antibiotics don't work on viruses Antibiotics target features of bacterial cells. Viruses have no cell to target, so they are unaffected — viral diseases are tackled with antiviral drugs and prevented with vaccines.
In one minute
Viruses are acellular, on the borderline of life — inert outside a host, active inside one.
Structure: nucleic acid (DNA or RNA) + protein capsid (± envelope); seen only by electron microscope.