A virus is simply a piece of DNA (or RNA) in a capsid coat that has the ability to enter a living cell, insert it's genetic material into that of the host cell, thereby taking it over an d instructing it to produce copies of itself (which then burst out of the cell to go and infect others). Accordingly, a virus is not alive, it is literally no more than an infective instruction to produce itself.
Single celled organisms are very much alive, and almost all are either prokaryote (i.e. bacteria) or protoctists (protozoans). because these are so small (and are generally referred to as germs) they can infect other multi-cellular organisms by getting inbetween the cells of that organiism and disrupting them (poisoning etc) thereby making the organism ''unwell''.
Multi-cellular organisms (anything larger than a germ!), including you and me, are simply huge collections of cells which all cooperate towards a common goal, all contributing something to the otherall function (brain cells for the brain, muscle cells form muscles etc).
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