522 FILTERABLE VIRUSES 



ceptible to cultivation. Nor does it seem to be a question of delicacy or sensitiveness, 

 because many of them are extremely resistant to chemical and physical agents. 

 Furthermore, no virus has been found multiplying free in nature. Therefore, the 

 viruses appear to be obligate parasites in the sense that their reproduction is depend- 

 ent upon living cells. Whether this reproduction occurs intra- or extracellularly is a 

 debated question. 



In view of the fact that viruses apparently multiply only in the presence of living 

 cells, it is necessary to consider what kinds of living cells best promote their repro- 

 duction, and what effect upon the cells is induced by this reproduction. In the first 

 place, a remarkable species specificity is exhibited by many viruses. Rous's sarcoma 

 grows only in chickens. Sanarelli's virus of infectious myxomatosis and Virus III are 

 active only in rabbits. The "salivary-gland virus" described by Cole and Kuttner af- 

 fects only guinea pigs. A wilt virus that attacks one kind of caterpillar is innocuous 

 for other caterpillars. Second, young cells seem essential for the activity of many 

 viruses. The bacteriophage multiplies only in the presence of young, growing bac- 

 teria. The activity of mosaic viruses is manifested only in young leaves. Virus dis- 

 eases usually attack insects in certain stages of development. In the higher forms of 

 life the activity of many viruses is also best exhibited not in old, undernourished, 

 sickly individuals, but in young, healthy ones. Lastly, some viruses, e.g., the virus of 

 rabies and poliomyelitis, exhibit a remarkable affinity for cells of certain tissues and 

 apparently can neither multiply nor produce signs of disease unless they come into a 

 close relation with these cells. 



It has been shown that some viruses multiply only in a restricted number of 

 hosts, that frequently this multiplication occurs only when the virus is in close rela- 

 tion with certain types of cells, and that young, actively growing cells play an im- 

 portant role in the infectiousness of many virus diseases. 



A word may be said in regard to the effects produced in cells by viruses. At first 

 the involved cells show a remarkable increase in size, often with amitotic division of 

 the nuclei. The increase in size gives one the impression that it is due to growth phe- 

 nomena and to imbibition of fluid. This process is spoken of as "ballooning degener- 

 ation." Eventually the cells die and go to pieces. This process is spoken of as "col- 

 liquation." Two forces seem to operate: one stimulating the cell, the other destroy- 

 ing it. Consequently, the picture produced by a virus disease is more or less dependent 

 upon which of these forces predominates. Chicken-pox, foot-and-mouth disease, vari- 

 ola, and lysis of bacteria by bacteriophage are diseases in which destructive agencies 

 predominate. Rous's sarcoma, contagious epithelioma, fowl leukemia, and warts are 

 diseases in which stimulating forces are dominant. Some observers' have attempted to 

 classify the cytotropic viruses under cytolytic and cytokinetic headings with subdi- 

 visions under each according to the type of cells involved. Such a classification is pre- 

 mature. 



It is not known whether the viruses multiply intra- or extracellularly. Neverthe- 

 less, they have a profound influence upon cells and cause remarkable changes within 

 them. This influence most likely accounts for the fact that in lesions caused by many 

 viruses intracellular changes assume appearances characteristic enough to be spoken 



' Pliilibert, A.: Ann. dc med., i6, 283. 1924. 



