252 



THE BIOLOGY OF DISEASE 



introduced into his circulatory system a virus, or serum. 

 Each disease has its own virus, as the vaccine in small- 

 pox, and this virus produces a mild form of the disease. 

 This causes the cells to become resistant to the germs or 

 microbes of the specific disease. Inoculation is being 

 widely used for the prevention of typhoid fever. All 

 soldiers are required to take this treatment. It would 

 be desirable for all people to become immunized against 



this disease, but those 

 who travel extensively 

 and thus have to drink 

 all kinds of water and 

 milk should certainly 

 undergo this treatment. 

 Vaccination and im- 

 munization reduce the 

 liability of death in case 

 the disease is acquired, 

 but they do not ab- 

 solutely prevent the disease. If a vaccinated or immu- 

 nized person gets an overwhelming number of germs, he 

 may have an infection of a slight kind. But the liability 

 of contagion is reduced to a minimum. 



192. Antitoxin. — We cannot say definitely why vaccina- 

 tion and immunization act as they do. It is known that 

 if a poison (toxin) produced during a case of diphtheria 

 is gradually introduced into the blood of a horse, a sub- 

 stance is produced which destroys the injurious effects of 

 the diphtheria poison. The serum from the blood of the 

 horse is called antitoxin, and may be preserved for use at 

 any time to destroy the influence of the diphtheria poi- 

 son. A given amount of this antitoxin is introduced into 

 the blood of the patient suffering from diphtheria, and 

 this usually counteracts the disease. This treatment has 



Figure 251. — Milk Diluted to j-^- 



Left-hand culture from clean milk ; 

 right-hand culture from dirty milk. 



