Viruses are obligate intracellular parasites consisting of genetic material (DNA or RNA) enclosed in a protein capsid, sometimes surrounded by a lipid envelope. They are not considered living organisms as they cannot replicate or carry out metabolic processes outside a host cell.
Basic Viral Structure
The viral genome can be either DNA or RNA, single-stranded or double-stranded, linear or circular, and segmented or non-segmented; viral genomes are significantly smaller than cellular genomes, typically 3-200 kb. The capsid is a protein shell composed of repeating subunits called capsomeres that protect the viral genome, and capsids exhibit icosahedral, helical, or complex symmetry. The envelope is a lipid bilayer derived from the host cell membrane, studded with viral glycoproteins (spikes) that mediate host cell attachment and entry. Matrix proteins are found in enveloped viruses between the capsid and envelope, providing structural integrity.
Capsid Symmetry
Icosahedral symmetry features twenty triangular faces forming a spherical shell, as seen in adenovirus, herpesvirus, and poliovirus, with capsomere numbers varying (e.g., 252 in adenovirus). Helical symmetry has capsomeres arranged in a helix around the genome, forming a rod-like or filamentous structure, as seen in tobacco mosaic virus, rabies virus, and Ebola virus. Complex symmetry describes viruses like poxviruses and bacteriophages that do not fit simple icosahedral or helical symmetry, often containing specialized structures such as tails and fibers.
Classification by Genome Type
The Baltimore Classification divides viruses into seven groups based on genome type and replication strategy. Group I consists of dsDNA viruses (e.g., adenovirus, herpesvirus); Group II consists of ssDNA viruses (e.g., parvovirus); Group III consists of dsRNA viruses (e.g., reovirus); Group IV consists of positive-sense ssRNA viruses (e.g., coronavirus, poliovirus); Group V consists of negative-sense ssRNA viruses (e.g., influenza, rabies); Group VI consists of ssRNA-RT retroviruses (e.g., HIV); and Group VII consists of dsDNA-RT viruses (e.g., hepatitis B virus).
Taxonomic Classification
In viral taxonomy, the order is the highest taxonomic rank (e.g., Mononegavirales, Herpesvirales). Family names end in -viridae (e.g., Picornaviridae, Coronaviridae, Retroviridae). Genus names end in -virus (e.g., Enterovirus, Betacoronavirus, Lentivirus). A species is defined as a monophyletic group of viruses with shared biological and genomic properties.
Viral Morphology
Naked (non-enveloped) viruses have their capsid directly exposed, making them resistant to drying, acid, and detergents, and they are typically transmitted via the fecal-oral route and fomites. Enveloped viruses are more sensitive to environmental conditions, heat, and disinfectants, and are typically transmitted through direct contact, blood, or respiratory droplets.
