Viral Structure and Basic Organisation

Viruses are acellular obligate intracellular parasites. They possess a genome and a protective structure but no independent metabolic machinery, no ribosomes, and no capacity to replicate outside a host cell.

• DNA or RNA: a virus contains only one type of nucleic acid, never both. This is the primary classification criterion (full Baltimore classification detail in Virology).
• Single stranded or double stranded: DNA viruses are usually double stranded (a few exceptions, e.g. parvoviruses, are single stranded); RNA viruses may be either.
• Segmented versus nonsegmented: most viral genomes exist as a single continuous molecule (nonsegmented). Some, notably influenza virus, carry the genome split across multiple separate segments, a structural feature that enables genetic reassortment when two different strains co-infect the same cell, the mechanism underlying influenza antigenic shift (full detail in Virology).

• The protein shell directly surrounding the viral genome, built from repeating protein subunits called capsomeres.
• Function: protects the genome from the extracellular environment (nucleases, desiccation, pH) and mediates initial attachment to host cell receptors in nonenveloped viruses.
• Genome plus capsid together are termed the nucleocapsid.

Lipid envelope:

• A lipid bilayer surrounding the nucleocapsid in enveloped viruses, acquired by budding through a host cell membrane (plasma membrane, nuclear membrane, or ER/Golgi membrane, depending on the virus).

Viral glycoproteins:

• Virus-encoded proteins embedded in the envelope, project outward as spikes. Mediate receptor binding and membrane fusion during entry, and are the principal target of neutralising antibodies.

Host derived membrane:

• The lipid component of the envelope itself is derived from the host cell, not virus-encoded. This host origin is directly responsible for the envelope's fragility.

• Icosahedral: a near-spherical geometric shape built from 20 triangular faces, the most common and most efficient capsid architecture for enclosing genetic material with minimal protein (e.g. adenovirus, herpesviruses, poliovirus).
• Helical: capsomeres arranged in a spiral around the nucleic acid, producing a rod-shaped or filamentous structure (e.g. rabies virus, Ebola virus, and the internal nucleocapsid of influenza before envelopment).
• Complex: a structure that does not fit either simple geometric category, often with additional external structures (e.g. poxviruses, bacteriophages with a head-tail architecture).

• This distinction has direct infection control implications: alcohol-based hand sanitiser reliably inactivates enveloped viruses but is far less effective against nonenveloped viruses, which is why outbreaks of norovirus in particular require soap-and-water hand hygiene rather than alcohol gel alone.

A generalised seven-step outline, with organism-specific detail developed fully in Virology.

1. Attachment: viral surface protein (glycoprotein in enveloped viruses, capsid protein in nonenveloped viruses) binds a specific host cell receptor, the principal determinant of host range and tissue tropism.
2. Penetration: virus enters the cell, by membrane fusion (enveloped viruses) or by endocytosis/direct injection (nonenveloped viruses, and bacteriophages respectively).
3. Uncoating: the capsid is degraded or opened, releasing the viral genome into the cytoplasm or nucleus.
4. Genome replication: the viral genome is copied, using host cell machinery, virus-encoded enzymes, or both, depending on the virus's Baltimore class.
5. Protein synthesis: viral mRNA is translated by host ribosomes into structural and nonstructural viral proteins.
6. Assembly: newly synthesised genomes and capsid proteins self-assemble into new virions.
7. Release: new virions exit the cell, by lysis (destroying the host cell, typical of nonenveloped viruses) or by budding through a host membrane (acquiring the envelope in the process, typical of enveloped viruses, and not necessarily lethal to the host cell immediately).