Molecular virology has developed very prominent tools to dissect viral functions by engineering infectious molecular clones with preselected modifications. Molecular virology is the study of virus on a molecular level. Viruses are submicroscopic parasites that replicate inside host cells. Viruses rely on their host to replicate and multiply. This is because viruses which are unable to go through cell division, as they are acellular–meaning they lack the genetic information which encodes the necessary tools for protein synthesis or generation of metabolic energy. Pathogenicity is the ability of one organism to cause disease in another. There is a specialized field of study in virology called viral pathogenesis in which it studies how viruses infect their hosts at the molecular and cellular level.