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.

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

[2] 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] This group of analysts defined multiple categories of virus.