Viruses vary greatly in size and structure. They contain a protein capsid that protects the viral nucleic acid genome, which can be DNA or RNA. Capsids have different symmetries like icosahedral, filamentous or head-tail shapes. Viruses replicate inside host cells by attaching, entering, releasing their genome, replicating it, assembling new viral particles, and exiting via lysis or budding. Their ability to highjack host cell machinery allows replication but also determines their pathogenicity and transmission between hosts.