Chapter 6: Viruses & Acellular Infectious Agents

Virology is the discipline dedicated to studying viruses, unique acellular infectious agents that rely entirely on host cell machinery for replication, existing either in an inactive extracellular state (virions) or as active nucleic acids intracellularly. A complete virion consists of a nucleocapsid, which includes the viral nucleic acid genome—structurally diverse and potentially double-stranded or single-stranded DNA or RNA—and a protein coat called the capsid. Capsids display defined symmetries, primarily helical (rod-shaped) or icosahedral (20 triangular faces), though some large viruses exhibit complex structures like the binal symmetry of T-even bacteriophages. Many animal viruses acquire a lipid envelope derived from the host membrane upon release, often containing viral proteins known as spikes (peplomers) that facilitate host cell attachment. The viral life cycle involves five generalized steps: attachment (adsorption) to specific host receptors (which dictates host tropism), entry into the cell (often via membrane fusion or endocytosis in eukaryotes), synthesis of viral components (where RNA viruses must carry or encode unique enzymes), self-assembly of new nucleocapsids, and release, typically by host cell lysis (common for phages and naked viruses) or budding (common for enveloped viruses). Bacterial viruses can be virulent (only lytic cycle) or temperate, opting for lysogeny where the viral genome (prophage) remains dormant, sometimes conferring new traits to the host via lysogenic conversion. In eukaryotic hosts, infections can be acute/cytocidal, chronic, latent, or result in transformation into malignant cells caused by oncoviruses disrupting normal cell-cycle control genes like proto-oncogenes and tumor suppressor genes. Viruses are cultivated in living systems, such as bacterial lawns (forming plaques), tissue cultures, or embryonated eggs, and their concentration is quantified indirectly through methods like the plaque assay (yielding PFU), hemagglutination, or the determination of infectious dose (ID50). Beyond viruses, the chapter covers simpler acellular agents: viroids (infectious circular ssRNA molecules that exclusively target plants, replicating using host enzymes but encoding no proteins) and prions (infectious protein-only particles, PrPSc, that cause fatal neurodegenerative diseases by converting the normal cellular protein, PrPC, into the aggregated abnormal form).