A viral disease (or viral infection) occurs when an organism's body is invaded by pathogenic viruses, and infectious virus particles (virions) attach to and enter susceptible cells.

Structural characteristics
Basic structural characteristics, such as genome type, virion shape and replication site, generally share the same features among virus species within the same family.

There are five double-stranded DNA families: three are non enveloped (Adenoviridae, Papillomaviridae and Polyomaviridae) and two are enveloped (Herpesviridae and Poxviridae). All of the non-enveloped families have icosahedral capsids.

There is one family of partly double-stranded DNA viruses: Hepadnaviridae. These viruses are enveloped.

There is one family of single-stranded DNA viruses that infect humans: Parvoviridae. These viruses are non-enveloped.

There are seven positive single-stranded RNA families: three non enveloped (Astroviridae, Caliciviridae and Picornaviridae) and four enveloped (Coronoviridae, Flaviviridae, Retroviridae and Togaviridae). All the non-enveloped families have icosahedral nucleocapsids.

There are six negative single-stranded RNA families: Arenaviridae, Bunyaviridae, Filoviridae, Orthomyxoviridae, Paramyxoviridae and Rhabdoviridae. All are enveloped with helical nucleocapsids.

There is one family with a double-stranded RNA genome: Reoviridae.

There is one additional virus (Hepatitis D virus) which has not yet been assigned to a family but is clearly distinct from the other families infecting humans.

There is one family and one genus of viruses known to infect humans that have not been associated with disease: the family Anelloviridae and the genus Dependovirus. Both of these taxa are non-enveloped single-stranded DNA viruses.

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