November 22, 1912] 



SCIENCE 



691 



vaccinia, hog-cholera and chicken-plague. 

 We can at present form no reliable con- 

 ception of the biology of this class of para- 

 site, although the virus of pleuropneu- 

 monia shows aiSnities with the bacteria, 

 while that of yellow fever that passes a 

 stage of its existence in mosquitoes prob- 

 ably belongs to the protozoa. It should be 

 remembered that we possess no criterion of 

 their presence other than the power to pro- 

 duce infection. Probably the list of these 

 pathogenic parasites would be increased if 

 methods were known for testing their sym- 

 biotic relations or cooperative effects with 

 the usual bacteria and protozoa. RousV^ 

 discovery of a filterable agent that causes 

 sarcomatous tumors in the fowl has opened 

 up new fields to exploration. "We can make 

 a rough guess as to their sizes since some 

 pass through thick filters, the pores of 

 which are smallest, while others pass the 

 more porous filters with larger interstices 

 only. Were the viruses as large as one 

 fifth the size of the influenza bacillus, they 

 would be beyond visibility with the most 

 powerful optical system of the modem 

 microscope. The dark-field microscope and 

 the instrument devised for employing, for 

 photographic purposes, the ultraviolet rays 

 of the spectrum, that has doubled the po- 

 tential power of the microscope, have failed 

 to bring them into view. On the whole 

 they resist drying well and show consid- 

 erable resistance to disinfecting agents. 



The ultramicroscopic viruses employ no 

 single means of effecting entrance into the 

 body. Some utilize insects to inject them 

 into the blood. Mosquitoes inoculate the 

 parasites of yellow fever and of dengue in 

 man, and the virus of horse-sickness among 

 animals; while flies inject the virus of par- 

 rot-fever; and worms and other insects, 



" Eous, Journal of Experimental Medicine, 1911, 

 XIII., 397; Journal of the American Medical As- 

 sociation, 1911, LVI., 198. 



through close contact with infected and 

 then with uninfected tobacco plants, dis- 

 seminate the parasite of mosaic disease. 

 The viruses of rabies, vaccinia and fowl- 

 pox gain entrance through skin wounds, 

 those of hog-cholera, foot and mouth dis- 

 ease and chicken-plague, by swallowing, 

 while the parasites of variola and of 

 pleuropneumonia are inhaled with air. 

 These are the main avenues but not the 

 sole routes of infection, since viruses that 

 ordinarily enter the body by the respira- 

 tory mucous membrane may occasionally 

 enter through a skin abrasion, etc. 



It is significant that upon recovery from 

 this class of infections a high and enduring 

 degree of immunity is left behind. We 

 have no knowledge of toxic substances, in 

 the common sense, being produced by the 

 filterable viruses, and therefore know noth- 

 ing of the formation of antitoxins or bodies 

 that neutralize poisons. The principles 

 upon which the immunity depends appear 

 to be chiefly microbicidal or substances 

 that act directly upon the living parasites 

 and destroy them. In some instances it 

 has been possible to produce an actively 

 immune state without at the same time 

 causing severe disease, by employing for 

 inoculation modified and weakened viruses 

 and viruses combined with immune sera 

 carrying the corresponding microbicidal 

 substances. Once a certain active im- 

 munity is obtained it can be heightened by 

 repeated injections of more active ma- 

 terials until a high degree is achieved. In 

 the same manner immune animals that 

 have recovered from disease are capable of 

 having this immunity reinforced by sub- 

 sequent injections of the active virus. 

 Blood taken from the immune animals has 

 been employed in practise in two ways: to 

 protect for a brief period exposed animals 

 from acquiring infection, and to bring 



