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.
Helper Dependent Virus