HOMO MILITAXS 



a species-foreign host drastically modifies the properties of viruses. 

 Thus, in monkeys the virus of yellow fever causes symptoms that show 

 a close resemblance to those of the human disease. Inoculated into 

 mice, the same virus causes a form of encephalitis that can be trans- 

 ferred in series in this animal. But if now the virus is reinoculated into 

 monkeys, it is not yellow fever that results; the virus retains its neuro- 

 tropic characteristics. 



This clearly shows that at any time we may be confronted with new 

 disease symptoms of man without this having to be ascribed to a de 

 novo creation of new parasitic forms. 



Besides, there exists another source of new diseases for man, viz., 

 an initiatory contact with forms of parasitism that have become estab- 

 lished elsewhere in the animal kingdom. Such a contact always offers 

 the opportunity of a transmission to a human host. A typical instance 

 of a new virus disease that appeared in this manner is psittacosis, first 

 described by Ritter in 1879, m which cockatoos or parakeets undoubt- 

 edly serve as the reservoir of the virus which is extremely contagious 

 and highly pathogenic for man. 



A second instance of a disease that probably originated recently was 

 first described in 1910 as the 'ground squirrel disease' in Tulare Coun- 

 ty, California. The causative agent turned out to be a hitherto un- 

 known bacterium, similar to though clearly distinguishable from the 

 bacterium of bubonic plague; it has been named Pasteurella tiilarensis. 

 Nine years later, and in a distant section of America, Utah, a party 

 that had been hunting wild rabbits and had skinned part of the booty 

 for food, took ill. In both the patients and the rabbits P. tiilarensis was 

 found. Since that time the disease, now known as tularemia, has been 

 encountered not merely in many parts of America, but also in Norway, 

 Sweden, the U.S.S.R., and Japan, often in the form of epidemics. 



The causative agents of these two new diseases, against which a 

 wholesale defense is still in its infancy, have been specifically included 

 in the experimental investigations, initiated during the war in Camp 

 Detrick, U.S.A., on the prevention of the spread of contagious dis- 

 eases. 



Although we may fervently hope that man will be spared the dis- 

 grace of bacteriological warfare, we Hollanders should feel humiliated 

 if we remember that it was one of our compatriots who was probably 

 the first to advocate this kind of warfare. It was Pieter van Woensel, 



41 1 



