helper virus

{{Distinguish|L-A helper virus}}

A helper virus is a virus that allows an otherwise-deficient coinfecting virus to replicate. These can be naturally occurring as with Hepatitis D virus, which requires Hepatitis B virus to coinfect cells in order to replicate. Helper viruses are also commonly used to replicate and spread viral vectors for gene expression and gene therapy.{{cite book|chapter=14 - Immunization Against Viral Diseases |vauthors=Graham BS, Crowe JE, Ledgerwood JE |page=403 |title=Fields Virology |edition=6 |veditors=Knipe DM, Howley PM |publisher=Wolters Kluwer Health |date=2007}}