A virus that by mutation has lost the ability to be replicated in the host cell without the aid of a helper virus. The virus particles (virions) contain all the viral structural components; they can attach, penetrate, and release their nucleic acid ribonucleic acid (RNA), deoxyribonucleic acid (DNA)) within the host cell.

However, since the mutation has destroyed an essential function, new virions will not be made unless the cell was simultaneously infected with the helper virus, which can provide the missing function. Only then will the cell produce a mixed population of new helper and defective viruses. Occasionally, when their nucleic acids become integrated in the DNA of the host cell, defective viruses persist in nature by propagation from mother cell to daughter cell.