Viruses

Viruses are small, non-cellular, intracellular parasites that consist, at the minimum, of a nucleic acid core and a protein capsid. Viruses are one of the most abundant biological entities and are known to infect an enormous variety of cells. Due to their simplicity in comparison to human cells, they are problematic to target in terms of human disease.

Because viruses are non-cellular and unable to maintain themselves, they are not considered organisms.

Structure
A viruses microstructure can be divided into three parts: 1) a nucleic acid core, 2) a protein capsid, and 3) a lipid bilayer envelope. Viruses also have spikes, but they are considered part of the outermost layer of the virus (the envelope or capsid, depending on whether the virus has an envelope or not).

The core functions as the viral genome. It is classified by the type of nucleic acid (DNA or RNA) and the number of strands (ss: single-stranded or ds: double-stranded). Double-stranded RNA and single-stranded DNA are known to occur only in viruses.

The capsid functions in protecting the viral genome and transmitting the genome into a host cell. Capsids are classified as closed shell, helical, or complex. Capsids are made of viral protein subunits called capsomeres. In closed shell capsids, capsomeres form isometric facets that result commonly in icsohedra (20-figured) arrangements. Other possible arrangements include cubic, polyhedral, and so forth. In helical capsids, capsomeres are arranged helically and results in a rod shape. Commonly, helical capsids have a hole in one end exposing the core. In complex capsids, both helical and closed shell arrangements may be found, as well as other structures or enzymes (e.g. the collar and tail pins of a bacteriophage).

Morphological Classification
Viruses can be divided dichotomously into DNA and RNA viruses, based on the type of nucleic acid in the core.

 DNA Viruses 
 * Herpesviruses
 * Herpes Simplex
 * Varicella-Zoster
 * Epstein-Barr
 * Papillomaviruses
 * Human Papillomavirus
 * Hepatitis B virus

 RNA Viruses 
 * Rhabdoviruses
 * Rabies
 * Picornaviruses
 * Poliovirus
 * Retroviruses
 * HIV
 * Influenza