HABITAT 957 



or of a completely avirulent variant of a parasitic virus. Barnard, however, by 

 ultra-violet photography has been able to detect the presence of minute, cultivable 

 bodies, about 150 m/i in diameter, in sterile tubes of serum broth. 



Many of the viruses in the animal body appear to show a particular affinity 

 for special tissues, such as nervous tissue or skin ; this resembles the affinity mani- 

 fested by many of the known bacteria for special tissues. Even, however, when 

 the lesions are confined to one tissue, the virus can frequently be demonstrated 

 in other parts of the body. There is evidence, too, that the tissue localization is 

 more apparent than real, depending on the mode of infection. Thus Ledingham 

 (1924) found that in rabbits the virus of vaccinia, which usually affects the skin, 

 was able to give rise to nodules on the peritoneum after direct inoculation into 

 the spleen or the abdominal cavity. As well as their selective tissue localization, 

 many viruses exhibit a species-specificity, giving rise to lesions only in one particular 

 species of animal. Thus Cole and Kuttner's salivary gland virus is active only 

 in guinea-pigs, Virus iii only in rabbits, and so on. On the other hand, there 

 are viruses, such as those of rabies and foot-and-mouth disease, which are pathogenic 

 not only to different species but also to widely separate groups of animals. Possibly 

 too much weight has been laid in the past on the species-specificity of the viruses. 

 It is now clear that most of the viruses are capable of infecting several different 

 species of animal under experimental conditions. 



Apart from the presence of a virus in a healthy carrier free from all clinical 

 symptoms of disease, it has been shown that a virus may remain latent in the 

 tissues after causing an initial infection. Thus, according to Gastinel and Reilly 

 (1928) the herpes virus can sometimes be demonstrated in the brain of guinea-pigs 

 that have recovered from a keratitis caused by inoculation of the cornea. Its 

 presence gives rise to no symptoms, but can be shown by inoculation of the brain 

 on to the cornea of a normal guinea-pig. It is possible that an attack of inter- 

 current disease, or some artificial procedure such as vaccination, may activate 

 such a latent virus, and cause it to give rise to clinical disease. In some virus 

 infections, such as yellow fever, there is reason to believe that the virus, after 

 causing an attack of the disease, remains latent in the tissues for years, if not 

 for the patient's whole life. By this means a lasting immunity is maintained 

 (see Rivers 1943). 



Whether the filtrable viruses occupy an intra- or an extracellular position in 

 the body is not certainly known, but the indirect evidence so far accumulated 

 suggests that their growth and multiplication occurs actually within the cells. The 

 rinderpest virus, for example, appears to be contained within the leucocytes ; by 

 centrifugalization of the blood, the virus is found to be concentrated mainly 

 in the leucocytic layer. Similarly with the virus of fowl-plague, Todd (1928) 

 found that in centrifuged blood the concentration of the virus was 100 times 

 greater in the leucocytic layer than in the clear plasma or the washed red 

 cells. Moreover there is evidence that for their multiplication the filtrable viruses 

 often prefer young newly-formed cells ; many of the viruses acting on the skin, 

 for example, give rise to lesions first along the lines of scarification, where repair 

 is taking place. Further evidence in favour of this view is that certain viruses, 

 e.g. Virus iii and vaccinia, have been found to grow in a transplantable rabbit 

 tumour, in which the cells are in process of active multiplication, and to survive 

 longer in the tumour than in the healthy tissues of the rabbit (Rivers and Pearce 

 1925). The observations of Perdrau and Todd (1936) on the lower susceptibility 



