A<unk>  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.<unk> es are also commonly used to replicate and spread viral vectors for gene expression and gene therapy.
Helper virus
Senecavirus
Coronavirus 
Coronavirus
Passenger virus
Arbovirus
Echovirus
Pandoravirus