690 



SCIENCE 



[N. S. Vol. XXXVI. No. 934 



proved to be, for when a portion of the 

 spinal cord of a recently paralyzed monkey 

 was made into an emulsion with sterile 

 distilled water, or simple saline solution, 

 and then centrifugalized to remove the 

 coarse suspended matter and afterwards 

 pressed through a Berkefeld earthenware 

 filter, which excludes ordinary cells, bac- 

 teria and protozoa, the clear liquid result- 

 ing was still capable of transmitting the 

 disease. The activity of the filtrate is very 

 great, since a fraction of a cubic centi- 

 meter still suffices to cause paralysis and 

 death. The only distinction to be noted 

 between the action of corresponding 

 amounts of the emulsion and filtered fluid 

 is that the former acts more quickly, as 

 would be expected from the fact that it 

 contains a greater number of the invisible 

 organisms. This difference is soon com- 

 pensated by the multiplication of those in 

 the filtrate so that the end result is the 

 same. By employing somewhat greater 

 quantities of the filtrate for inoculation 

 the incubation period of the disease can be 

 made the same as that following the use of 

 the emulsion. The disparity is strictly a 

 quantitative one, since the filters retain a 

 part of the minute organisms in their pores 

 and thus reduce the number escaping with 

 the filtrate. The greater the quantity of 

 protein matter present in the fluid the 

 fewer the parasites that pass the filter, and 

 merely because the large protein molecules 

 themselves tend to be held in the pores 

 and thus render them impervious for the 

 minute organisms. For this reason, also, 

 fluids containing small numbers of the fil- 

 terable parasites, but still sufficient to 

 cause infection in the crude state, may fail, 

 when filtered, to produce disease merely 

 beeaiise those retained by the filter so far 

 reduce the numbers as to bring them below 

 the surely infecting dose. This reduction 



sometimes leads to another effect; namely, 

 the slight degree of infection that forms 

 the starting point of active immunization. 

 By building upon such a beginning a high 

 and enduring state of immunity has been 

 achieved. 



The first filterable parasite was discov- 

 ered by Loeffler fourteen years ago, in the 

 fluid lymph obtained from the vesicles of 

 cattle suffering from foot and mouth dis- 

 ease. At the present time eighteen dis- 

 eases are known that are believed on good 

 ground to be caused by this class of minute 

 living organisms. One alone among them 

 is on the verge of visibility — the parasite 

 causing pleuropneumonia of cattle. It 

 alone has certainly been obtained in arti- 

 ficial culture. The methods of artificial 

 cultivation need still to be worked out ; and 

 once they are discovered it is a safe predic- 

 tion that control over the diseases pro- 

 duced by ultra-microscopic parasites will 

 be quickly increased. The degree of inf ec- 

 tivity of certain of the parasites — or vi- 

 ruses, as they are also called — is almost 

 fabulous. One thousandth of a cubic cen- 

 timeter of a filtered 2.5 per cent, suspen- 

 sion of a spinal cord of a paralyzed monkey 

 suffices to cause infection and paralysis in 

 another monkey; 0.020 of a cubic centi- 

 meter of infected lymph suffices to produce 

 foot and mouth disease in a healthy calf, 

 and the blood of fowl suffering from 

 chicken plague is still active after being 

 diluted 1,000 million times with water. 



Three affections of human beings are 

 contained among the eighteen diseases 

 caused by filterable viruses: They are 

 yellow fever, dengue and poliomyelitis. 

 With one exception, the mosaic disease of 

 tobacco, the remaining fourteen are mal- 

 adies of domestic animals and include 

 among them foot and mouth disease, horse- 

 sickness, cattle-plague, sheep-pox, rabies, 



