Viruses are too small to be seen with a light microscope can be seen with an electron microscope cannot be cultured outside their hosts.I. Introduction to Viruses Although viral diseases are not new, the viruses themselves could not be studied until the 20th century. In 1886, the Dutch chemist Adolf Mayer showed that tobacco mosaic disease (TMD) was transmissible from a diseased plant to a healthy plant. In 1892, in an attempt to isolate the cause of TMD, the Russian bacteriologist Dimitri Iwanowski filtered the sap of diseased plants through a porcelain filter that was designed to retain bacteria.o Iwanowski expected to find the microbe trapped in the filter; instead, he found that the infectious agent had passed through the minute pores of the filter.o When he infected healthy plants with the filtered fluid, they contracted TMD. The first human disease associated with a filterable agent was yellow fever. Advances in the molecular biological techniques in the 1980s and 1990s led to the recognition of several new viruses, including human immunodeficiency virus (HIV and SARS-associated coronavirus. Israeli acute paralysis virus became a national concern in 2006, when it killed up to 90%of the pollinating bees in the U.S. hives.—This new virus was 1st seen in bees in 2002 andseems to have been in the U.S. since then.II. General Characteristics of Viruses 100 years ago, researchers could not imagine submicroscopic particles, thus they described the infectious agent as contagium vivum fluidum (contagious living fluid) By the 1930’s, scientists had begun using the word virus, the Latin word for poison, to describe these filterable agents. The nature of these viruses remained elusive until 1935, when Wendell Stanley, an American chemist, isolated tobacco mosaic virus, making it possible for the 1st time to carry out chemical and structural studies on a purified virus.—At about the same time, the invention of the electron microscope made it possible to see viruses. Are viruses living organisms?—The answer is AMBIGUOUS.NO: Viruses are not considered to be living organisms because they are inert outside living host cells.YES: Viruses are considered alive when they multiply in the host cell they infect.Once viruses enter a host cell, the viral nucleic acids become active, and viral multiplication begins.1Clinical point of view—Viruses are alive because they can cause infection and disease like pathogenic bacteria, fungi, and protozoa.How do we define a virus?Viruses were originally distinguished from other infectious agents because:1). Viruses are especially small (filterable: pass through bacteriological filters)2). Viruses are obligatory intracellular parasites—they absolutely require living host cells in order to multiply.However, both of these properties are shared by certain small bacteria such as some rickettsias.Nonenveloped viruses—do not have plasma membranesEnveloped animal viruses—acquire plasma membrane from host cells after they bud from the host cells which becomes the viral envelope.The truly distinctive features of viruses are now known to relate to their simple structuralorganization and their mechanism of multiplication. Viruses Contain a single type of nucleic acid, either DNA or RNA (never both) Contain a protein coat that surrounds the nucleic acid.—sometimes is enclosed by an envelope of lipids, proteins, and carbohydrates. Are obligatory intracellular parasites—absolutely require living host cells in order to multiply. Multiply inside living cells by using the synthesizing machinery of the cell. Cause the synthesis of specialized structures that can transfer the viral nucleic acid to other cells. Have few or no enzymes of their own for metabolism. i.e., viruses lack enzymes for protein synthesis & ATP generations*In order to multiply, viruses must take over the metabolic machinery of the host cell.Host Range of a Virus Is the spectrum of a host cells the virus can infect.i.e., there are viruses that can infect invertebrates, vertebrates, plants, protists, fungi, & bacteria.However, most viruses are able to infect specific types of cells of only one host species.In rare cases, viruses cross the host-range barrier, expanding their host range.2Bacteriophages, or phages: Viruses that infect bacteria.The particular host range of a virus is determined by:1). The viruses requirement for its specific attachment to the host cell.2). The availability within the potential host of cellular factors required for viral multiplications.In order for the virus to infect the host cell: The outer surface of the virus must chemically interact with the specific receptor sites on the surface of the host cell. The two complementary components are held together by weak bonds, such as hydrogen bonds. The combination of many attachment and receptor sitesleads to a strong association between the host cell and virus.For some bacteriophages: The receptor site is part of the cell wall of the bacterial host. In other cases, the receptor site is part of the fimbriae or flagella of the bacterial host.For animal viruses: The receptor sites are on the plasma membranes of the host cells.Viral Size Is determined with the aid of electron microscopy. Viruses range from 20 to 1000 nm in length Different viruses vary considerably in size Most viruses are quite a bit smaller than bacteria Some of the larger viruses (such as the vaccine virus) are about the same size as some very small bacteria (such as the mycoplasmas, rickettsias, and chlamydias).III. Viral StructureVirion Is a complete, fully developed, infectious viral particle. Is composed of nucleic acid. Is surrounded by a protein coat that protects is from the environment. Is a vehicle of transmission from one host cell to another.Viruses are classified by differences in the structures of these protein coats.Nucleic Acid A virus can have either RNA or DNA (never both) The nucleic acid can be single-stranded or double-stranded.3 i.e., there are viruses with—Double-stranded SNA, single-stranded DNA, double-stranded RNA, single-stranded RNA the nucleic acid can be linear or circular (depends on the virus) in some viruses (such as the influenza virus) the nucleic acid is in several separate segments. Percentage of nucleic acid in relation to protein:Is about 1% for InfluenzavirusIs about 50% for certain