The Position of Viruses in the Biological Spectrum
Viruses are infectious particles that invade every known type of cell. They are not alive, yet they are able to redirect the metabolism of living cells to reproduce virus particles.

Viral replication inside a cell usually causes death or disease of that cell.

The General Structure of Viruses
Virus size range is from 15 nanometers to 200 micrometers.

Viruses are composed of an outer protein capsid enclosing either DNA or RNA plus a variety of enzymes. Some viruses also exhibit an envelope of cell membrane over the capsid.

Viral particles are called virions.

Viruses reproduce through five major events that turn a cell into a virus factory which synthesizes viral components, then assembles them into complete virion units. The new virus particles leave the cell host by lysing the cell or by budding.

Bacterial viruses or bacteriophage inadvertently incorporate bacterial DNA into the virion in place of viral DNA, thus providing a means of genetic transfer in bacteria.

Lysogeny is a condition in which viral DNA is inserted into the bacterial chromosome and remains inactive for an extended period of time. It is replicated as part of the bacterial genome every time the bacteria divides.

Unlike bacteriophage, animal viruses vary in their methods of adsorption, penetration, site of replication, and method of exit form host cells.

Animal viruses can persist in host tissues as chronic latent infections which can reactivate periodically throughout the host's life. Some persistent animal viruses are oncogenic.

Techniques in Culturing and Identifying Animal Viruses
Animal viruses must be studied in some type of host cell environment such as laboratory animals, bird embryos, or tissue cultures.

Tissue cultures are cultures of host cells grown in special sterile chambers containing correct types and proportions of growth factors using aseptic techniques to exclude unwanted microorganisms.

Virus growth in tissue culture is detected by the appearance of plaques.

Medical Importance of Viruses
Viruses are easily responsible for several billion cases of infections each year. It is conceivable that many chronic diseases of unknown cause will be connected to viral agents.

Other noncellular agents of disease are the so-called slow viruses, which are not viruses at all, but protein fibers called prions, virinos, extremely small lengths of protein-coated nucleic acid; and satellite viruses, which require larger viruses to cause disease.

Diagnosis of viral disease agents is done directly through culture, electron microscopy, immunofluorescence microscopy and by the PCR technique. Indirect diagnosis is done by testing patient serum for host antibody to viral agents.