First, what do we mean by “alive”?
Biology has no single universally accepted definition of life that settles every borderline case. Scientists often consider a cluster of characteristics: cellular organisation, energy use, regulation, growth, response to the environment, reproduction and evolution. No single item is a perfect test in every case.
What is a virus made of?
A complete virus particle, called a virion, carries genetic information—DNA or RNA—inside a protein coat called a capsid. Some viruses also have a lipid envelope. A virion is not a cell: it has no ribosomes and does not maintain its own metabolism.
What happens when a virus meets a cell?
A virus cannot make new virus particles entirely by itself. It must enter a suitable host cell and use the cell’s machinery, together with viral proteins and genetic instructions, to produce components that can be assembled into new virions. The details vary considerably among virus groups.
Outside a host cell, a virion does not feed, grow or reproduce. Inside a host, viral genes can redirect cellular activity toward making more virus.
Think of the distinction: a virus has instructions, but it depends on a living cell to execute them.
But viruses evolve. Doesn’t that make them alive?
Viral populations change over generations. Mutations arise as genomes are copied; recombination and, in some viruses, reassortment can also generate variation. Natural selection can favour variants that reproduce more successfully in a particular environment.
Evolution is a defining feature of life, but it is not by itself a complete test. Viruses evolve, yet their reproduction depends on cells.
So, are viruses alive?
The answer depends partly on which characteristics a definition treats as essential. If cellular organisation and independent metabolism are required, viruses do not qualify as living organisms. If the emphasis is on genetic information, reproduction within a host and evolution, viruses share important features with life.
A careful description is that viruses are evolving biological entities, but are not autonomous living organisms. The question reveals as much about how we define life as it does about viruses.
Further reading
Koonin and Starokadomskyy, “Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question,” Studies in History and Philosophy of Biological and Biomedical Sciences (2016). Read the paper.
NCBI Bookshelf: Medical Microbiology, “Structure and Classification of Viruses” (Gelderblom).