Chapter 17: Viruses

Viruses consist of nucleic acid genomes enclosed within protein shells called capsids, composed of repeating subunits known as capsomeres that form helical, icosahedral, or complex geometries. Many viruses acquire a lipid bilayer envelope derived from host cell membranes and decorated with viral glycoproteins that facilitate host cell recognition and entry. Viral genomes display remarkable diversity, ranging from small DNA or RNA molecules with minimal genes to larger genomes containing thousands of genetic elements organized in linear or circular configurations. The replication cycle varies fundamentally between viral types. Bacteriophages undergo either lytic cycles that rapidly destroy host cells through viral assembly and release or lysogenic cycles in which viral DNA integrates into the bacterial chromosome as a prophage and replicates passively alongside host genes until induction triggers lytic progression. Bacteria defend against phage infection through mutation, restriction enzyme systems, and adaptive CRISPR-Cas mechanisms that store molecular memories of past infections. Animal viruses exhibit diverse replication strategies including entry via membrane fusion, exploitation of host ribosomes and enzymes for genome replication and protein synthesis, and release through budding or lysis. Retroviruses such as HIV employ reverse transcriptase to synthesize DNA copies of their RNA genomes, integrating these sequences as proviruses into host chromosomes for persistent infection and immune evasion. Viruses likely originated from escaped cellular nucleic acids including plasmids and transposons, while giant viruses such as Mimivirus and Pandoravirus possess genomes and cellular machinery that challenge conventional viral definitions. As disease agents, viruses cause significant infections in animals, plants, and humans, driving the evolution of preventive vaccines and therapeutic antiviral compounds. Emerging viruses including influenza, Ebola, Zika, and West Nile virus arise through rapid mutation in RNA-dependent polymerases, host range expansion, and zoonotic transmission events. The chapter concludes by examining prions, which are infectious proteins that propagate disease through self-perpetuating conformational misfolding of normal cellular prion proteins, causing invariably fatal neurodegenerative conditions.