Viruses are the simplest, smallest particles to possess the fundamental properties of living systems. (Also, as we know only too well, they are responsible for many plant and animal diseases.) However, viruses are so simple that they occupy a ‘twilight zone’ at the border between the living and the non-living. Viruses are essentially short segments of genetic material – RN A or DN A – protected by ‘overcoats’ of protein molecules. By itself a virus can do nothing. It cannot grow, or reproduce, or carry out any metabolic processes. But this same virus, when it infects a living cell, seizes control of the cell’s metabolic machinery and can then do all the things required of a living organism.
So, is a virus alive or dead? We can regard it as a parasite which lives off another organism without destroying it (although some viruses do destroy their hosts). The virus makes its host cell perform living functions for it. Perhaps the definition of life given above, whereby a system must be capable of performing a basic set of functions, still holds for viruses.