A parasite can be considered to be the device of a nucleic acid which allows it to exploit the gene products of other nucleic acid–the host organisms. In this view, all parasites are ‘molecular parasites’. But it is interesting to restrict our attention to nucleic acids which do not encode organisms, as these live in a purely molecular world which lacks emergent features such as fangs and ovipositors. Viruses and transposons are molecular parasites in this sense. Most viral nucleic acids do code for some proteins, such as replicases and the protein shell in which they travel between their cellular oases. Some, however, do not even have a shell and code for nothing at all–these are the ‘viroids’ (Reisner & Gross, 1985), the smallest parasites in the world.
