240 FLEAS, FLUKES AND CUCKOOS 



The Viruses 



The viruses are a group of organisms which are nearly all too small 

 for the eye to perceive even with the aid of a microscope, although they 

 can sometimes be photographed with the aid of the electron microscope. 

 Their presence is revealed only when they stimulate some noticeable 

 reaction in the host. They have never been cultivated outside the living 

 cell, and the exact nature of a virus is a matter for speculation. They 

 may be minute micro-organisms somewhat resembling very small 

 bacteria, progressively degraded by a parasitic form of life which they 

 have pushed to the utmost limits of specialisation. On the other hand 

 there is another theory supported by some powerful evidence, which 

 suggests that viruses are not living organisms at all but chemical sub- 

 stances — huge nuclear proteins which multiply by so-called auto- 

 catalysis. Like bacteria and parasitic Protozoa the viruses can reach 

 their hosts by contact between infected individuals or contaminated 

 materials including food and water, or by insect carriers; some are air- 

 borne and others infect their hosts by unknown means. 



At least one virus disease, psittacosis, sometimes called ornithosis, 

 apparently possesses a certain news value. Consequently when a keeper 

 in the parrot house at the zoo contracts it or an old lady with budgeri- 

 gars dies of the infection, the British pubHc learn of the occurrence along 

 with the latest murder story and the football results. The causative 

 agent is a virus which invades and destroys the reticulo-endothelial 

 cells, giving rise to clinical symptoms resembhng influenza but often 

 compUcated by pneumonia. It is rather larger than the ordinary 

 filterable viruses and falls mid-way between them and the Rickettsia- 

 like bacteria. Parrots are on the whole more susceptible than other 

 birds, but pigeons, finches, gulls, ducks, pheasants and fulmars also 

 suffer from the disease. In the years 1933 to 1937, there were autumn 

 epidemics of pneumonia among the human inhabitants of the Faeroe 

 Islands, the cause of which was traced by Rasmussen to a wide- 

 spread infection among juvenile fulmars which were used by the 

 islanders to supplement their ordinary diet. Nearly six times as many 

 women as men were infected and it was assumed that they inhaled the 

 virus along with a fine dust which is hberated when birds are plucked. 

 During the war it was discovered that carrier pigeons in Britain were 

 not infrequently infected with psittacosis, but there were no cHnical 

 symptoms and it was a type which did not apparently attack man. It 



