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]Untill now, we enough spoke about diseases in a lot of articles, and the new methods to defeat'em. In some topics, viruses were the responsible of the infections ( read, for instance: http://phyrtual.org/en/project/1675/news/gold-nanoparticles-against-viruses ), but the question is: what are viruses?

It's very difficult answering to this questions. For many years, viruses had been classified between unliving and living matter, in which just in living cell viruses get all the features of a living organism. Discovered about 1892 and 1898 by the Russian Ivanowsky and the Dutch Beijerink, who proved that "Tobacco Mosaic" is due to a particles smaller than other known microorganisms.

Viruses are, in fact, non-cellular microorganisms which can only replicate within animal, microbial or plant cells. They are made up proteic coat, called capsid, which contain a core of nucleic acid, which could be made of DNA or RNA, in single or double filament.

A virus hasn't an own methabolism: it depends on entirely on its host cell for energy, for precursor molecules and for all essential enzymes not encoded by the viruses. The viral nucleic acid inserts itself in host's DNA. During its replication, the host's proteic syntesis incudes also the virus proteins: the reproduction is started.

Virus particles are assembled inside the host cell and, one time built, viruses leave the cell by destroying it, and infecting other near cells.

Some animal illnesses casued by viruses are: common cold, influenza, AIDS, herpes, hepatitis, polio and rabies; some viruses, called oncogenes, are also implicated in the development of cancer.

Virus can be inactivated by extreme levels of pH, by relatively mild heat treatment and by UV light. They cannot be defeated by antibiotics, for this one every viral desease only must be healed by organism's immune response ( see http://phyrtual.org/it/project/1675/case-study-free-style/the-immne-response )

Unlike viruses, viroids are just made up RNA and lack an envelope and capsid. Viroids trigger many troubles in agricoltural, by infecting potatoes, tomatoes and some fruit trees. Prions, instead, are just toxic proteins which are able to transmit diseases. These rogue proteins direct the host to create abnormal proteins that cause serious neurological disease in animals and humans. Mad Cow Desease is caused by a prion.