Vector
A replicon that is useful for cloning DNA fragments so that they can be amplified or transferred to other cells. Common cloning vectors are derivatives of natural plasmids, phages, or viruses.

Viable
The ability of an organism to grow and divide.

Virion
An infectious virus particle.

Virulence
The relative ability of an organism to cause disease.

Virulence factor
Any gene product which enhances the ability of an organism to cause disease.

Virulence gene
Slang for a gene encoding a virulence factor.

Virulent phage
A bacteriophage which always grows lytically.

Virus
A small, infectious, obligate intracellular parasite. The virus genome is composed of either DNA or RNA. Within an appropriate host cell, the viral genome is replicated and uses cellular systems to direct the synthesis of other viral components. Progeny viruses are formed by de novo assembly from the newly synthesized components within the host cell. Transmission of the progeny viruses occurs by release from the host cell, and infection of new host cells.

Vitamin
An organic compound that is required in relatively small amounts for growth of an organism. Vitamins often function as coenzymes.

VSP repair
"Very short patch" DNA repair. A DNA repair mechanism that corrects G-T mismatches arising from deamination of 5-methyl cytosine in enteric bacteria. A short DNA fragment including the mismatch is excised and resynthesized.