Topic overview

Start with the big picture

Viruses contain either DNA or RNA within a protein capsid, with some also surrounded by a lipid envelope. They depend on host cells and replicate by assembling components rather than dividing. Genome type, capsid symmetry, envelope status, and replication site inform classification; viral replication proceeds through recognizable stages, and inclusion bodies can support histologic diagnosis. Laboratory approaches include molecular, antigen, culture, microscopic, and serologic methods. Prions are misfolded proteins without nucleic acid that can convert normal prion protein into a pathogenic form and cause transmissible spongiform encephalopathies. Viroids are circular RNA without a capsid and are plant pathogens. These differences help explain diagnostic and infection-control considerations.