a virus is a submicroscopic entity possessing only one nucleic acid and capable of self replication only within specific host cells. Some viruses do possess one or more enzymes but not one has a single complete enzyme system, hence their dependence on host cells. Two groups of agents which are not quite viruses possess certain characteristics common to viruses. These are the Rickettstales, generally accepted as bacteria, and the Psittacosis—Lymphogranuloma-venereumTrachoma or PLT group which occupy the no-man's land between viruses and bacteria. Since both of these groups will be dealt with later only the "true viruses"1 will be considered in this chapter. Although the viruses superficially appear to form a homogenous group considerable differences exist. Some are obviously of a fundamental nature while others have been artificially made by man. A résumé of these differences with respect to size, physical structure, biochemical structure and growth should illustrate the artificial nature of the man-made divisions into plant, bacterial, vertebrate and arthropod infecting viruses
The size and physical structure of the infectious virus particle, or virion, can be considered together because in many ways they are related. The smallest viruses have a simple structure consisting of an inner core of nucleic acid and an outer coat of protein whereas the largest Poxviruses have a very complex structure. Viruses can be divided into four main categories from their appearance in the electron microscope.The "spherical" viruses consist of a central core of nucleic acid surrounded by a layer, or capsid, composed of regular protein sub-units called capsomeres. The "rod-shaped" viruses take the form of a hollow helix with the nucleic acid contained within the helical protein coat which again is made up from regular sub-units. The large complex viruses have an internal structure similar to the complete spherical and rod-shaped viruses but they are enclosed within one or more membranes. The classical "tadpole-shaped" bacteriophage or bacterial viruses
By the use of extremely sensitive electron microscopy, especially the phosphotungstic acid negative staining technique,13 the fine structure of the capsomeres is being determined. The hollow "rod-shaped" viruses have so far been found only among those viruses infecting plants. Again the "rod" is in fact a helical structure made up of regular capsomeres. This structure can best be explained by comparing it with a spiral staircase, having a central well and where each step represents a capsomere. The size and shape of these capsomeres differs with each virus as does the overall diameter, the length of the particle, the size of the central hole and the distance separating each turn. In the case of tobacco mosaic virus (TMV) the distance between each turn is 2-3 πιμ and X-ray diffraction has revealed that the diffraction pattern is only the same at every third turn.14 From this it was determined that for every three turns on the helix there are 49 capsomeres, which, for a length of 300 ναμ gives a total of about 2130 capsomeres
Each capsomere is grooved on each side, matching similar grooves on the neighbouring capsomeres. These grooves form a helical tube running throughout the virus particle. The nucleic acid is situated within this helical tube. There is such a tremendous variation in the length and diameter of the "rod-shaped" plant viruses that the dimensions and number of capsomeres making up each helix could be virtually unlimited
The structure of the complex viruses such as the ArbovirusesHerpesviruses, Poxviruses, Myxoviruses and several insect viruses will be found in the relevant chapters but a brief summary is necessary at this point. Some, including the Herpesviruses have an internal icosahedral structure similar to the "spherical" viruses but this is enclosed within one or more membranes which are often partially destroyed during fixation for electron microscopy.16»17 In the case of Herpesvirus the number of capsomeres in the capsid of the internal structure is 162. The remaining viruses are even more complex. In each case the internal structure resembles that of TMV but in this instance the hollow helix is coiled on and around itself and is enclosed within several membranes.18»19'20 These viruses range in size from a diameter of 38 χημ for yellow fever17 to the brick-shaped vaccinia particles measuring 300 χημ by 375 m^.19 Finally, the classical bacteriophage particle is tadpole-shaped with a polyhedral head and a narrower cylindrical tail.21-24 All bacteriophage were once thought to have this shape but it is now known that some have the same morphology as the "spherical" viruses. Phage T2, one of the largest of the tadpole-shaped variety, has a head size of 65 x 95 ναμ and a tail size of 25 x 100 χαμ
