What Is a Virus?

Viruses are built from short sequences of nucleic acid, either DNA or RNA wrapped in a protein shell. Until the invention of the electron microscope, it was impossible to visualize a virus. The first viruses to be visualized were bacteriophage, which appeared to have a head and tail-like structure. Only the nucleic acid entered the bacterial cell through the tail. Animal viruses were described as spherical or rod-shaped; they were bound to receptors and were taken up by the cell. After the crystallization of the tobacco mosaic virus, there was much discussion as to whether viruses were “living” organisms; the controversy continues to this day. Although viruses were defined in part on the basis of size and filterability, viruses much larger than the traditional viruses have recently been isolated. Studies of viral replication indicate that most viruses self-assemble as a result of interactions between the viral proteins to form a viral capsid that interacts with the nucleic acid to form the whole. The viral replication cycle and synthesis is presented in this chapter. Viral classification into a Linnaean scheme has been proposed, but newer methods using nucleic acid homologies are changing classification. Viruses are spread in the human population by various means, including airborne particles, fecal-oral contact, clothing, insects, and contact with other animals (zoonosis).