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.

Examples include the common cold, gastroenteritis, COVID-19, the flu, and rabies.

Basic structural characteristics, such as genome type, virion shape and replication site, generally share the same features among virus species within the same family.^{[citation needed]}

Human-infecting virus families offer rules that may assist physicians and medical microbiologists/virologists.^{[citation needed]}

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.^{[citation needed]}

This group of analysts defined multiple categories of virus. Groups:^{[citation needed]}

The clinical characteristics of viruses may differ substantially among species within the same family: