228 RAVENEL— THE CONTROL OF [April is, 



was almost certainly clue in large part to vaccination. As the troops 

 had considerable freedom in visiting the city, this conclusion is 

 rendered all the more certain. 



Preparation of the Vaccine. 



The method of preparation varies slightly in dififerent labora- 

 tories, but the following is probably most often followed. 



Pure cultures of the typhoid bacillus are grown on slanted agar, 

 preferably in flat bottles, which give a large surface for culture. At 

 the end of forty-eight hours the bacilli are scraped ofif and suspended 

 in normal salt solution. The suspension is then heated for one hour 

 to a temperature of 53° C, preferably in a water bath, after which 

 it is standardized by comparing it with normal blood. Equal parts 

 of normal human blood and the suspension of bacteria are mixed, 

 and oftentimes diluted in order to facilitate counting. Spreads from 

 the mixture are made on slides, stained, and a large number of fields 

 (usually one hundred) examined, and both red blood cells and bac- 

 teria are counted. The average number of blood cells per field and 

 the average number of bacteria per field are then compared. The 

 normal blood count is taken at five million red cells for each cubic 

 millimeter. Knowing this, it is easy to determine the number of 

 germs per cubic centimeter. The vaccine is then diluted with normal 

 salt solution until the mixture contains one thousand million bacilli 

 per cubic centimeter. If it is to be sent out to physicians in general 

 practice, it is better also to make a further dilution of the suspen- 

 sion to five hundred million per cubic centimeter for the first injec- 

 tion so that the size of the dose may be kept uniform while the num- 

 ber of bacteria contained in the dose is varied. The vaccine may be 

 preserved for considerable lengths of time by the addition of one 

 fourth per cent, of lysol, or carbolic acid. When kept in a cool and 

 dark place its properties are maintained uninjured for at least three 

 months. 



It is also advised that the material should not be used until it is 

 three weeks old, as freshly prepared vaccine apparently is more apt 

 to g:ive severe local reactions than that which is older. 



