Helper virus

A helper virus is a virus used when producing copies of a helper dependent viral vector which does not have the ability to replicate on its own. The helper virus is used to co-infect cells alongside the viral vector and provides the necessary enzymes for replication of the genome of the viral vector.

Hepatitis B virus (HBV) is an example of a helper virus that helps Hepatitis D virus (HDV) (a replication defective, helper dependent ssRNA virus) to replicate -- as HDV requires the surface antigen of HBV (HBsAg) for the encapsidation of its own genome. The envelope proteins on the outer surface of HDV are entirely provided by HBV.