BACTERIA 139 



bating the attack of disease germs, by making them more suscep- 

 tible to the white corpuscles. It seems as if the opsonin in the blood 

 can be increased by the injection of dead germs, and this method 

 is sometimes used to produce immunity to certain diseases. 



Acquired Immunity. In some diseases, it seems as if the fact of 

 having had the attack and successfully overcoming it, had provided 

 the body with such ability to supply that particular antitoxin 

 that the person seldom has the disease again, as for example in 

 the case of measles and whooping-cough. The body has been 

 trained, as it were, to oppose that kind of attack and this is called 

 a condition of " acquired immunity." 



ARTIFICIAL PROTECTION 



Vaccination. From this it follows that if one has a mild attack 

 of a serious disease, he may develop sufficient antitoxin strength 

 to oppose the dangerous form, somewhat as a sham battle pre- 

 pares the soldier to protect himself in the real engagement. This 

 fact is the basis of vaccination which is the inoculation of a well 

 person with a mild form of smallpox, by which he becomes able 

 to resist the attack of this terrible disease. (Smallpox is due to 

 a one-celled animal germ, not a bacterium.) In a similar way 

 protection is obtained against typhoid fever and hydrophobia. 

 Weak doses of the toxins of these diseases are administered, so 

 that the body gradually increases its antitoxin defences and be- 

 comes immune to fatal attack. Some people oppose vaccination 

 because when improperly performed, other germs are introduced 

 and serious illness follows but this is a very rare occurrence. Be- 

 fore vaccination was practiced 95 per cent of all people had small- 

 pox, thousands died and all were scarred for life. Then it was 

 one of the plagues of the world, whereas it is now one of the rarest 

 of diseases. 



Antitoxins. Another method of helping our bodies to repel 

 germ attack is by administration of the antitoxin directly. In 

 vaccination the body learns to make its own, but there are cases 

 where a child is too weak to do this and the actual antitoxin is 



