A virus, structurally, is not much: genetic material — DNA or RNA — wrapped in a protein shell called a capsid, sometimes with a lipid envelope stolen from a previous host cell around the outside. That's close to the whole parts list. There are no ribosomes to build proteins, no enzymes to generate energy, no machinery to do anything at all. Outside a living cell, a virus doesn't eat, move under its own power, or metabolise in any sense biologists usually mean by the word. It just sits, chemically inert, until it happens to encounter a cell it can get into.
Hijacking, not eating
Once a virus attaches to a cell whose surface receptors happen to match its own surface proteins, it forces or tricks its way inside, sheds its protein coat, and releases its genetic material into the cell. From that point on, every step of replication is carried out by the host's own machinery: the host's ribosomes read the viral genetic instructions and build viral proteins; the host's enzymes copy the viral genome; the host's own energy supply powers all of it. The virus contributes instructions. The cell does the metabolic work — its own, redirected entirely toward making more virus, often until the cell ruptures and releases the new copies to repeat the cycle elsewhere.
Why "is it alive" is a genuinely contested question
Viruses sit right on a definitional line biology hasn't fully agreed how to draw. They carry heritable genetic material and evolve by natural selection, which looks like life. They can't independently metabolise, can't maintain their own internal chemistry, and can't reproduce without commandeering a cell that can, which looks like the opposite. Depending on which criterion a given textbook treats as essential — independent metabolism, or heritable replication and evolution — viruses land on different sides of the same question, and biologists genuinely disagree, not out of carelessness but because "life" itself doesn't have one universally accepted definition to check a virus against.
What we're still unsure about
Where viruses came from in the first place is still unresolved, with several serious competing hypotheses rather than a settled account: that they're stripped-down remnants of once free-living cells that lost everything but their genes over evolutionary time; that they arose as fragments of cellular DNA that gained the ability to move between cells independently; or that they co-evolved alongside the earliest cellular life from the start, rather than descending from it. The 2003 discovery of giant viruses — some carrying more genes than certain bacteria — complicated the picture further instead of resolving it, reopening exactly the debate about where the boundary between virus and cellular life actually sits.
This sits inside Viral Structure & Replication, one of seven topics in Microbiology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.