A viral disease (or viral infection, or infectious disease) occurs when an organism's body is invaded by pathogenic viruses, and infectious virus particles (virions) attach to and enter susceptible cells.[1]

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.

Pragmatic rules

Human-infecting virus families offer rules that may assist physicians and medical microbiologists/virologists.

As a general rule, DNA viruses replicate within the cell nucleus while RNA viruses replicate within the cytoplasm. Exceptions are known to this rule: poxviruses replicate within the cytoplasm and orthomyxoviruses and hepatitis D virus (RNA viruses) replicate within the nucleus.