Viruses (virii) are strands of DNA or it's complimentary form, RNA, usually coated by a protein shell called the capsid. Viruses lie at the very cusp of life - they cannot replicate themselves on their own and need to coerce a host to make copies of themselves. Because of this they need to get inside the cells of living organisms - animals, plants, fungi, plasmodiums or even bacteria.

It is important to distinguish between viruses and bacteria. Bacteria are fully functional, single-celled life-forms which are capable of replicating themselves on their own. Whilst both viruses and bacteria can cause diseases, the ways in which they do this are very different. Unlike bacteria, viruses do NOT respond to antibiotics. Viruses are many times smaller than both bacteria and human cells. Although many bacteria cause diseases, most of them do not live inside the bodies of other organisms and do not cause diseases.

There are three kinds of virus,

• DNA viruses which carry regular DNA much like that in the nucleus of human cells. These include the herpes family, smallpox, and the papilloma family of viruses which cause warts and benign and malignant cancers (it is estimated that the papilloma viruses may cause 16% of cancer in women and 10% of all cancers worldwide).

• RNA viruses which carry a substance called RNA. In animal and plant cells, proteins are synthesised from DNA via an RNA stage. RNA viruses parasitise the normal functioning of the cell by operating in this RNA phase of protein synthesis. Retro-viruses are themselves sub-divided into
• plus-stranded RNA viruses, eg. polio virus and the flavivirus family which causes yellow fever, dengue, and various forms of encephalitis.
• negative-stranded RNA viruses, eg. influenza, measles, mumps and rabies viruses.
• double-stranded viruses, eg. Colorado tick fever virus.

• Retro viruses which carry RNA but copy this back into DNA when they are operating with the organism's cell. Examples of retro-viruses are HIV which causes AIDS and HTLV-1 which infects T-cells and causes Tropical Spastic Paraparesis, a differential diagnosis of Multiple Sclerosis.

Many viruses have been implicated in the development of multiple sclerosis. Currently, two Herpes viruses, Human Herpes Virus 6 (HHV-6) and Epstein-Barr virus (EBV), are the focus of attention but it may very well be that viruses have nothing whatsoever to do with the etiology of MS.

Upper respiratory chest infections, some of which are associated with viral infections, have been shown to be positively correlated with MS relapses. However, these may exacerbate MS by activating the immune system through the release of pro-inflammatory cytokines and may not be directly related to the disease.

Virus links:
What The Heck is a Virus?
Virology
A dictionary of virology
The Big Picture Book of Viruses

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MS Glossary
All About Multiple Sclerosis