text: The identification of the genetic basis for the causative agent of a disease can be an important component of understanding its effects and spread. Location and content of structural genes can elucidate the evolution of virulence, as well as provide necessary information for treatment. Likewise understanding the specific changes in structural gene sequences underlying a gain or loss of virulence aids in understanding the mechanism by which diseases affect their hosts.
  For example, Yersinia pestis (the bubonic plague) was found to carry several virulence and inflammation-related structural genes on plasmids. Likewise, the structural gene responsible for tetanus was determined to be carried on a plasmid as well. Diphtheria is caused by a bacterium, but only after that bacterium has been infected by a bacteriophage carrying the structural genes for the toxin.
  In Herpes simplex virus, the structural gene sequence responsible for virulence was found in two locations in the genome despite only one location actually producing the viral gene product. This was hypothesized to serve as a potential mechanism for strains to regain virulence if lost through mutation.
  Understanding the specific changes in structural genes underlying a gain or loss of virulence is a necessary step in the formation of specific treatments, as well the study of possible medicinal uses of toxins.