Virus

Microscopic view of a virus.

A virus is a microscopic, non-cellular, infectious agent that has two characteristics: It has genetic material (either DNA or RNA) inside a protective protein coat (called a "capsid"). Uniquely, viruses may contain single or double stranded nucleic acid. All other organisms are based on double stranded DNA.

To reproduce itself, a virus needs to infect a living cell. The genetic material is inserted into the cell, and the reproductive machinery of that cell is then effectively "hijacked": the cell starts to produce components of new virus particles. These components spontaneously self-assemble and the new virus particles can then infect other cells, where the same process happens.

Classification

Viruses are classified based on several characteristics:
The type of nucleic acid (DNA vs RNA)
The strandedness of that acid (single strand vs double strand)
The content of their protein capsids
The hosts they are able to infect.
The presence or absence of an envelope around the capsid (a lipid layer derived from host cell membranes with embedded proteins).

Viruses which infect bacteria are referred to as "bacteriophages".

Forms of replication

Broadly speaking, there are two types of replicative cycles viruses undertake; lytic and lysogenic. The cycles are described below.

Lytic
The first step to viral replication is entry into the cell. The virus uses proteins in either its capsid or envelope to bind to a protein on the surface of the cell. Each virus can only use certain proteins which influences which cells they are able to infect (a property called 'tropism'). One inside, the virus must shed its protein coat and prepare to be replicated. The process of replication will differ based upon whether or not the virus is DNA based or RNA based. Following replication virus particles self assemble and leave the cell, either by bursting through the membrane or budding off leaving the membrane intact.

Lysogenic
Lysogenic replication, on the other hand, involves the virus integrating into the host genome. The virus is then replicated every time the genome replicates. In times of stress the virus genome can excise itself from the host genome and enter the lytic cycle.

Are viruses alive?

There is disagreement on whether or not viruses should be considered alive since they do not share many of the processes that other forms of life have. They do not feed, respire, grow, reproduce through cell division, or move under their own power. Viruses also do not have a cell structure, despite having genes.

Examples

Examples of viruses that infect humans include the influenza virus, the common cold, HIV, smallpox and polio.

Viruses that attack bacteria are called bacteriophages.

Immunities

Viruses are not targeted by antibiotics, the drugs used to kill bacteria, and so use of antibiotics for viral infections is usually inappropriate. There are, however, many antiviral drugs used in cases of viral infection; most notably acyclovir, interferon, and ribavarin.

Other uses

Malicious computer programs are generally called viruses, as well. A computer can get "infected" with a computer virus, often without the user initially being aware of this, and the virus can then damage the information on the computer and spread to other computers.