322 ACUTE EXANTHEMATA. 



nucleus of the cell is stained a dark purple, the cell-bodies 

 pink, and the peculiar bodies a light uniform purple. 



Immunity : One attack of smallpox confers an immunity 

 of from ten to twenty years' duration, although not infre- 

 quently the immunity persists for a lifetime. A mild attack 

 of the disease will confer immunity against severe attacks. 

 This fact forms the basis of vaccination, which is now prac- 

 tised almost universally. 



The virus used for vaccination, or vaccine, is obtained from 

 the pustules of cowpox. The contents of the pock are rubbed 

 up with glycerin. Human lymph was also used at one time, 

 but its use has been abandoned for the bovine lymph, because 

 of the possibility of its being contaminated with tuberculosis 

 and syphilis. The bovine vaccine is also more easily obtained. 

 The vaccine is placed on the market either in the form of 

 an ivory point which is coated on both sides with the dried 

 vaccine ; or in capillary tubes. Good vaccine will keep in 

 sterilized capillary tubes for three months, although some 

 tubes deteriorate before that time. 



Vaccine always contains bacteria, very few of which are 

 pathogenic, however. These organisms usually disappear 

 from the lymph within three or four months. For this 

 reason it is not well to use too fresh a lymph, because it may 

 contain staphylococci, which will induce severe local suppu- 

 ration around the vaccinated area, or even a fatal pyaemia. 



Recent investigations by Funck show that vaccinia is 

 caused by Sporidium vaccinale, a parasitic protozoon. Animals 

 inoculated with this organism developed both vaccinia and 

 variola. These organisms were found in the lymph, in the 

 smallpox vesicle, and also in sterile glycerinated vaccine. 



When examined in the hanging drop two forms of this 

 organism are seen. One, a cyst-form filled with spores, and, 

 second, free spores. The spore-cysts are round or oval, and 

 about 25 fjt in diameter. The spores are irregular in outline, 

 highly refractive and motile, measuring from 1 to 3 /j. in 

 diameter. Larger bodies, which look like epithelial cells filled 

 with the organism, are also seen occasionally. 



Sporidium vaccinale can be obtained in pure culture by 

 plating in agar a few drops of sterile glycerinated. vaccine. 



