CONTAGIOUS DISEASES 373 



the illness ; but as a result of this, the person is protected 

 from the dangerous disease of smallpox. 



When the virus that causes cowpox is injected into the 

 blood, the body makes a substance, called antibody, which 

 offsets the effect of the poison and the person recovers. 

 This antibody, thus made, remains in the system and is 

 effective in neutralizing the injurious effects of any small- 

 pox microorganisms that may enter the body. The time 

 that this immunity lasts varies with different people, the 

 average time being about seven years, although some slight 

 degree of immunity is afforded for a lifetime from a single 

 vaccination. 



Effect of vaccination on the death rate. Epidemics of small- 

 pox are so rare in this country to-day that people are apt to 

 become careless about the matter of vaccination, overlooking 

 the fact that it is vaccination which has made this freedom 

 from smallpox epidemics possible. Statistics furnish indis- 

 putable evidence of the great blessing that vaccination has 

 proved to mankind in saving hundreds of thousands of lives. 



Before the discovery of vaccination, smallpox was one of 

 the most common diseases, even more common than measles 

 to-day. The great majority of people at some time had the 

 disease. In the city of Boston in 1721, over one half the 

 people had the disease and one thirteenth of the population 

 died of it. Nine years later, there was another epidemic 

 which was almost as severe. During the centuries past 

 smallpox has been one of the greatest scourges throughout 

 the world, sweeping off people by the thousands. It has 

 been estimated that 60,000,000 people died of smallpox in 

 Europe during the eighteenth century. 



Now an epidemic of smallpox is of very rare occurrence 



in those countries where vaccination is commonly practiced, 



and the number of deaths from this disease is very small. 



Records that have been kept of the deaths from smallpox 



. show that on the average the percentage of those who die 



