By many definitions of life, viruses don't make the cut. They consist of:
- a nucleic acid (DNA or RNA)
- encased in a protein capsule called a capsid,
- which, in some viruses, is coated by a lipid envelope (derived from one of the host cell's membranes)
In other words, they are the only type of organism not to be cellular. They also don't appear to have machinery for the absorption of nutrients, for the excretion of waste products, or for almost any metabolism at all. They are fundamentally a chunk of DNA or RNA surrounded by a rudimentary covering.
They get away with this, of course, by hijacking the resources of an honest, hard-working cell. Why have your own mitochondria when you can use someone else's? All other organisms, even prokaryotes, are bafflingly complicated assemblies of prodigious complexity - all of which is simply stolen and turned against the organism by a virus.
The virus can therefore be functionally divided into two aspects: the instructions that it wants the host cell to follow (i.e. the DNA or RNA) and the way of getting the instructions into the right place in the cell (i.e. the capsid and/or envelope). The host cell is of course blind, and simply follows whatever instructions it gets from DNA/RNA in the belief that these instructions come from the cell's 'own' DNA. Viruses exploit this weakness by pretending to be just this 'honest' DNA/RNA, and the cell's complicated machinery dutifully follows the instructions of the virus.
What do these instructions say? On the whole, they simply say: replicate me by building more virus particles (called virions). The host cell therefore recreates the original virus particle, nucleic acid plus capside, thousands and thousands of times over. The virions then exit the cell to infect others like it. Sometimes they cause the host cell to burst, scattering the tens of thousands of virions far and wide, but sometimes the virus forces the cell to export it in a more dignified manner, often collecting its envelope from the cell membrane on the way out.
It is interesting to note that although viruses are the simplest type of organism, they could not have been the first, at least in their present form. They depend entirely on already extant cells, which must have preceded them, to do the hard work. As I said, they are such efficient parasites that they have outsourced all of life barring a few orders, to the point where they are not even considered alive anymore. Remarkable!
Image from: Harrison's Image Library