296 APPLIED BIOLOGY 



This causes the human tissues to produce some opposing 

 substance which effectually prevents the disease smallpox for 

 many years, the length of immunity varying with individuals. 



No scientific man questions that vaccination against 

 smallpox has been one of the means (isolation and disinfec- 

 tion are others) which has made smallpox one of the rarest 

 diseases; and that it should be practiced whenever, in a 

 particular locality, there are cases which may have spread 

 infection widely. For example, when a case appears in a 

 school, all teachers and pupils in that school should be imme- 

 diately protected by vaccination performed by competent 

 doctors. 



Protective Inoculation. Similar to vaccination is protec- 

 tively inoculating by giving weakened doses of toxins. 

 Pasteur discovered that if the bacteria of cattle anthrax, or 

 those of chicken cholera, be grown in pure cultures in test- 

 tubes, the toxins get weaker as the cultures grow older. In 

 other ways, also, it is possible to weaken toxins of bacteria. 

 Now, if weakened toxins are injected first, there is a mild 

 attack of the disease. Then a stronger toxin will produce 

 no more effect ; and using in succession in a series of days or 

 weeks, stronger and stronger toxins, the animal into which 

 they are injected finally becomes unable to take the disease 

 and is said to be protectively inoculated. Many recent 

 experiments on thousands of soldiers seem to prove that it 

 will be possible to inoculate against typhoid in a similar way. 

 The destructive cholera of pigs and the distemper which 

 annually kills thousands of dogs are being studied in an 

 attempt to find a successful way of protectively inoculating. 



Hydrophobia. The treatment for this disease is another 

 well-known example of protective inoculation. Persons 

 bitten by dogs believed to have rabies go to a Pasteur Insti- 

 tute, and receive frequent injections of weakened toxins 

 obtained from the spinal cords of rabbits which have died 

 with the disease. The result of these weakened doses, which 



