Viruses, Viroids, and Prions

Obligate Intracellular Parasites

Biological agents (such as viruses) that cannot replicate or survive outside of a cellular host and cannot be treated or cured with existing medical therapies.

'Katie Var.'

A camellia variety demonstrating a virus-variegated bloom, illustrating how plant viruses can induce phenotypic variegation in host flora.

Viroids

The smallest known pathogens, composed of naked, circular, single-stranded RNA molecules that do not encode proteins yet replicate autonomously when introduced into host plants.

Virion

A complete viral particle consisting of a nucleic acid core (DNA or RNA) surrounded by a protein coat (capsid), and occasionally an outer lipid envelope.

Capsid

The protein shell that encloses and protects the viral nucleic acid core.

Capsomeres

Identical individual protein subunits that bond together to assemble the capsid and confer structural symmetry to the virus.

Embryonated Egg Inoculation Sites

The distinct anatomical compartments within a fertilized chicken egg utilized to cultivate viruses in a laboratory: the chorioallantoic membrane, amniotic cavity, allantoic cavity, and yolk sac.

Helical Symmetry

A viral capsid self-assembly pattern in which protein subunits and viral nucleic acid are arranged spirally around a central axis to form a rod or cylinder.

Icosahedral Symmetry

A viral capsid self-assembly pattern in which protein subunits assemble into a symmetric shell covering the nucleic acid core, characterized by 2-fold, 3-fold, and 5-fold axes of symmetry.

Bacteriophage T4

A complex binal-symmetry virus measuring $80 \times 225\,\text{nm}$ that infects bacteria and is composed of a DNA-containing icosahedral head, a contractile tail sheath, and tail fibers.

Viral Envelope

An outer phospholipid bilayer surrounding the capsid in many mammalian viruses, derived directly from the host cell's plasma membrane during viral maturation and release.

Viral Tropism

The specificity of a virus for infecting particular host cell types determined by precise binding between viral surface components and host cell receptors (and co-receptors).

Virion Uncoating

The step in viral replication involving the disassembly and removal of the capsid, which can take place at the plasma membrane, inside the cytoplasm, or docking at the nuclear membrane.

Virion Maturation

The viral life cycle phase involving intracellular trafficking that moves newly made viral components to specific intracellular sites for assembly into complete particles.

Virion Assembly

The process of packaging the viral genome inside the capsid, which occurs either concurrently while the viral genome is synthesized or by inserting the genome into pre-formed capsids.

Budding vs. Lysis

The two primary release mechanisms of newly formed virions: non-enveloped viruses typically release via host cell lysis, whereas enveloped viruses typically exit by budding through host membranes.

Lytic Infection

A viral infection cycle wherein virions continuously replicate until the host cell fills with progeny and bursts, resulting in cell death.

Lysogenic (Latent) Infection

An infection cycle in which the viral genome incorporates into the host cell chromosome, persisting long-term without immediately killing the host cell (e.g., HIV, Hepatitis B and C, HPV).

Prophage

Bacteriophage DNA that has integrated into the circular bacterial chromosome via genetic recombination during the lysogenic cycle.

Eclipse Period

The early interval during an acute infection when free virions attach and penetrate host cells, resulting in a temporary drop in detectable infectious extracellular virions prior to cell burst.

Parvoviridae

A viral family of small ($18\text{--}25\,\text{nm}$), nonenveloped, single-stranded DNA viruses, exemplified by Human parvovirus B19, which causes Fifth disease and anemia in immunocompromised patients.

Poxviridae

A family of very large ($200\text{--}350\,\text{nm}$), complex, brick-shaped enveloped dsDNA viruses that cause smallpox (variola), cowpox, and molluscum contagiosum.