- All viruses consist of a viral core which possesses the viral nucleic acid genome as well as a proteinacious capsid which surrounds the core
- Some viruses also possess a lipid membrane covering termed the "Viral Envelope".
- The viral core consists of the viral genome together with associated proteins which package the genome. All viral genomes consist of nucleic acids but there is wide variety in their chemical composition and arrangement. As discussed further in Viral Classification, the genome may consist of DNA or RNA and be arranged in double stranded or single stranded form. Rather than float freely with the possibility of becoming tangled, viral genomes are neatly packaged within the core by proteins termed "Nucleoproteins".
- The viral capsid is a highly ordered proteinacious covering which surrounds and protects the viral core. Because of their ordered nature, viral capsids can display specific symmetric geometries which can be used to classify viruses. For some viruses (termed "Unenveloped Viruses") the viral capsid interacts with the outside environment and mediates attachment to host cells whereas other viruses (termed "Enveloped Viruses") are surrounded by a lipid membrane discussed next. Viral capsids tend to be fairly tough structures and most unenveloped viruses can infect host cells even after passing through relatively harsh environmental conditions such as the alimentary tract.
- As discussed, some viruses are surrounded by a lipid membrane and are thus termed "Enveloped Viruses". Viral envelopes are derived from the lipid membrane of host cells as the virus buds from the infected cell. While replicating inside host cells viruses generally direct viral proteins to sites of viral budding at the host cell membrane thus allowing the shed virus's envelope to possess viral proteins. Because the membrane of enveloped viruses is what interacts with the environment, these "Viral Envelope Proteins" are the components which direct attachment of the virus to uninfected host cells. Viral envelopes, like all lipid membranes, are relatively weak structures and are easily destroyed by harsh environmental conditions such as drying and detergents. Consequently, enveloped viruses tend to be transmitted only through moist environments such as respiratory droplets, secretions, and sexual contact. Finally, the space between the Viral Capsid and the Viral Envelope is known as the "Tegument" and some viruses possess specialized proteins that float around in the tegument and assist in the infectious process.