General Virology

Viruses are acellular, obligate intracellular infectious agents. Each virion is built from a nucleic acid genome, either DNA or RNA but never both, enclosed within a protein coat, with or without a surrounding lipid envelope depending on the virus.

Viruses possess no ribosomes, no independent metabolic machinery, and no capacity for ATP generation. This puts them in a fundamentally different biological category from any free-living, independently metabolising cell, bacterial or otherwise.

Because viruses lack ribosomes and independent metabolism, they cannot replicate outside a living host cell under any circumstances. Every step of viral protein synthesis, and for most viruses every step of genome replication, depends entirely on hijacking host cell machinery.

This obligate dependence is why viruses cannot be grown on artificial culture media of the kind used for bacteria. Laboratory propagation instead requires live cell culture systems, embryonated eggs, or a susceptible living host.

It is also the central conceptual obstacle in antiviral drug design. Because so much of viral replication literally is host cell machinery running under viral direction, achieving selective toxicity, killing the virus without harming the host cell, is intrinsically harder than for antibacterial drugs, which can exploit prokaryotic-specific targets entirely absent from human cells.

The virion is the complete, infectious viral particle as it exists outside a host cell. At minimum it consists of a genome (nucleic acid) and a capsid (protein coat), with an optional surrounding envelope depending on virus type.

Genome plus capsid together are termed the nucleocapsid.

A virus carries DNA or RNA, never both, within a single virion. Genomes are further subclassified by strandedness, single or double stranded, and, for RNA viruses specifically, by sense, positive or negative. Genome sense determines whether the viral RNA can be translated directly by host ribosomes on entry or must first be transcribed into a translatable copy, a distinction with major consequences for how quickly a virus can begin producing protein once inside a cell.

The capsid is the protein shell directly surrounding the genome, built from repeating protein subunits called capsomeres. These subunits self-assemble into one of three overall symmetry patterns: icosahedral, helical, or complex, each associated with characteristic virus families and structural properties.

The envelope is a lipid bilayer present in some but not all viruses. It is acquired from a host cell membrane during viral budding and is studded with virus-encoded glycoproteins that project outward from its surface.

Whether a virus carries an envelope has major downstream consequences for environmental stability, disinfectant susceptibility, and characteristic transmission route, since a lipid envelope is structurally fragile compared to a protein-only capsid and is readily disrupted by drying, heat, and lipid solvents such as alcohol and soap.

Structural proteins are incorporated into the finished virion itself: capsid proteins, envelope glycoproteins, and, in some enveloped viruses, matrix proteins forming a supporting layer between the nucleocapsid and the envelope.

Nonstructural proteins are not packaged into the final virion but are essential for replication inside the infected cell. This category includes viral polymerases, proteases, and integrases depending on virus type, along with regulatory proteins that control the timing and level of viral gene expression.

Some viruses package essential enzymes directly inside the virion itself, rather than relying on synthesis after entry. This is necessary specifically when an enzyme is needed immediately on cell entry, before any viral protein synthesis has yet had a chance to occur. The RNA-dependent RNA polymerase of negative-sense RNA viruses and the reverse transcriptase of retroviruses are the two clearest examples of this principle.

Host range describes the specific range of host species, and within a host the specific cell types, that a given virus is capable of infecting. It is determined predominantly by the availability of a compatible host cell surface receptor for viral attachment, and secondarily by the presence of host intracellular factors required for the subsequent steps of replication.

Host range can be narrow, restricted to a single species or even a single cell type, or broad, spanning multiple species. A broad host range is a prerequisite for the kind of zoonotic spillover events that periodically introduce a new virus into the human population.

Tissue tropism describes the specific tissue or cell type within a susceptible host that a given virus preferentially infects. As with host range, this is determined predominantly by receptor distribution, though tissue-specific factors required for efficient replication once entry has occurred also play a role.

Tissue tropism directly predicts clinical presentation, and this relationship is the single most useful organising idea in virology as a whole. A virus with respiratory epithelial tropism causes respiratory disease. A virus with neuronal tropism causes neurological disease. A virus with hepatocyte tropism causes hepatitis. A virus with CD4 T lymphocyte tropism causes progressive immunodeficiency. Knowing a virus's tropism, in most cases, is enough to predict the broad shape of the disease it produces before any further detail is known.

Cytopathic effects are the visible or measurable changes produced in an infected host cell as a direct or indirect consequence of viral infection. Historically these effects were a key tool for detecting viral growth in cell culture, since a cytopathic effect visible under light microscopy indicated successful viral replication even before the specific virus had been identified.

The underlying mechanisms producing these effects, including cell lysis, syncytium formation, and inclusion body formation, reflect distinct ways a virus can injure or kill the cell it infects, and different virus families characteristically favour different mechanisms.

Viruses are classified predominantly by genome type and structure, an approach known as the Baltimore classification, which organises viruses into groups based on the specific pathway their genome takes to produce mRNA. This is the primary classification axis used throughout virology because genome class predicts so much of a virus's basic replication biology in one step.

A secondary classification layer groups viruses by taxonomic family, which tends to track shared structural and biological features within a genome class, such as segmentation, envelope presence, and characteristic symmetry.

Despite substantial molecular variation between genome classes, essentially all viruses follow the same general replication sequence: attachment, entry, uncoating, genome replication, protein synthesis, assembly, and release. The specific enzymology and cellular location of each step differs considerably depending on whether the virus is a DNA virus, a positive-sense RNA virus, a negative-sense RNA virus, or a retrovirus, but the overall shape of the sequence holds across the board.