Virus and virus-like diseases of grapevine in Hungary

A virus is an infectious agent, often highly host-specific, consisting of genetic material surrounded by a protein coat. The word is from the Latin virus referring to poison and other toxious substances, first used in English in 1392. Virulent, from Latin virulentus (poisonous), dates to 1400. A meaning of “agent that causes infectious disease” is first recorded in 1728, before the discovery of viruses by Ivanovsky in 1892. The adjective viral dates to 1948. The term virion is also used to refer to a single infective viral particle. Viruses infect virtually every life form, including humans, animals, plants, even fungi, bacteria and fytoplasmas. So small that they cannot be seen by a light microscope, viruses range in size from about 30 nanometers to about 450 nanometers and are between 100 to 20 times smaller than bacteria. Known viruses have been assigned to about 1600 species in more than 100 different families. Hundreds of other viruses remain unclassified due to lack of information. All standard viruses share a general structure of genetic material, or viral genom, and a protein coat, called a capsid. The viral genome is made of either dezoxyribose nucleic acid (DNA), the genetic material found in plants and animals, or ribonucleic acid (RNA), a compound plant and animal cells use in protein synthesis (Table 1). Viruses are not strictly free-living, as they cannot reproduce on their own. Instead, they use host cell machinery to make both the viral genome and capsid of the newly formed viruses, or virions. Virus particles (known as virions) consist of two or three parts: the genetic material made from either DNA or RNA, long molecules that carry genetic information; a protein coat that protects these genes; and in some cases an envelope of lipids that surrounds the protein coat when they are outside a cell. The shapes of viruses range from simple helical and icosahedral forms to more complex structures. The broad category of viruses also includes unusual infective agents that are missing one or more components of standard viruses. These unconventional viruses include viroids, which exist as circular RNA molecules that are not packaged. The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids – pieces of DNA that can move between cells – while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity.