Wikipedia: Biological virus

A virus (from the Latin virus, referring to poison and other noxious things) is a small particle which can infect other biological organisms.
The term "virus" usually refers to those particles which infect eukaryotes?, whilst "bacteriophage" or "phage" is used to describe those infecting prokaryotes. Typically these particles carry a small amount of nucleic acid (either DNA or RNA) surrounded by some form of protective "coat" consisting of protein, or protein and lipid.

One of several possible viral classifications :

Class I double stranded DNA
Class II single stranded DNA
Class III double stranded RNA
Class IV positive single stranded RNA itself acting as mRNA.
Class V negative single stranded RNA used as a template for mRNA synthesis.
Class VI positive single stranded RNA with a DNA intermediate in replication but and in mRNA synthesis.

The protective coat normally also enables the infective process which can occur by a variety of different mechanisms. The practical upshot all off of these is that the host cells replication machinery is hijacked to create more of the virus particles hence completing the life cycle. Viruses are somewhere between being living and non-living. They can reproduce and show inheritance, but are reliant on the complex enzymes of their hosts, and in many ways can be treated like ordinary molecules (for instance, they can be crystalized). Whether or not they are "alive" it is clear that they are obligate parasites, and have no form which can reproduce independant of their host. Like most parasites they have a specific host range, sometimes specific to one species (or even limited cell types of one species!) and sometimes more general.

Examples of diseases caused by viruses include AIDS which is caused by HIV, and cold sores which are caused by Herpes? Simplex. Recently it has been shown that cervical cancer is caused at least partly by papillomavirus (which causes papillomas, or warts), representing the first significant evidence in humans for a link between cancer and an infective agent.