infkctiox and infectious agents 39 



Importance of Host Factors in Determining Virulence. — 

 Falk* (1928) has objected to the use of the term virulence "to 

 denote the absolute capacity of a parasite to cause disease." He 

 points out that virulence is not primarily a characteristic of the 

 parasite but "varies reciprocally as resistance or immunity." In 

 other words the relative resistance of the host is a factor. The 

 Leptospira icterohemorrliaqiac is not virulent for the wild rat, 

 which is resistant to it, but is highly virulent for guinea pigs partly 

 because they are very susceptible. 



Aggressins. — Bail attributed virulence to certain biochemical 

 products formed either within the infected tissue or in cultures. 

 He named these chemical substances "aggressins." It has been 

 shown that if CI. chauvei, the causal agent of blackleg in cattle, 

 is washed with sterile saline until freed of all traces of culture 

 media and products of growth, it loses its virulence. The latter 

 is restored, however, when the washed organisms are mixed with 

 sterile filtrates of the original virulent culture. Scott (1931) has 

 recently reviewed the literature on aggressins quite extensively. 



Nature and Characteristics of Viruses. — Prior to 1935, very 

 little was known about viruses other than that they are much 

 smaller than bacteria, many being filtrable agents, and that they 

 require living host cells for their reproduction ; they cause specific 

 diseases and recovery from such diseases is usually accompanied 

 by a lasting immunity. From the standpoint of pathology it was 

 well established that in many virus infections characteristic cell 

 inclusion bodies are formed. 



In 1935 Stanley obtained the virus of tobacco mosaic disease in 

 crystalline form. While Stanley at first thought that the virus 

 protein is a crystalline globulin, Bawden and Pirie (1940) present 

 chemical analyses of six types of viruses which indicate that they 

 are nucleoprotcins. This view is also supported by the work of 

 Rischkaw (1940), Mcintosh (1940) and Daranyi (1940). It is 

 well established that the virus protein is biochemically and anti- 

 genically different from the proteins of the host. In regard to 

 size the viruses appear to exhibit, according to j\IcIntosh, prac- 

 tically a continuous series varying from the smallest 20 mix to the 

 largest which is 250 mu. According to Laidlaw (1938) the size 



♦From "A Theory of Microbic Virulence," by I. S. Falk in The Newer Knowl- 

 rdoe of Bacteriology and Immunology edited by E. O. Jordan and L. S. Falk. 

 Reprinted by permission of University of Cliicago Press. 



