428 GENERAL SCIENCE 



ering. The danger of spreading scarlet fever lasts as 

 long as the dead skin is " peeling off," a process which 

 continues after the patient has otherwise recovered. 



Smallpox. Smallpox is one of the eruptive diseases, 

 the sores being of a loathsome nature and leaving the 

 disfiguring " pock marks " if great care is not taken. This 

 disease was once a terrible scourge, and even up to the 

 beginning of the nineteenth century almost everybody 

 expected to have it. 



The first attempt at preventing the disease came with 

 the discovery that milkmaids who had become infected 

 with cowpox (smallpox of the cow) either did not get 

 smallpox or had very light cases. This fact in 1796 led 

 Sir Edward Jenner to experiments which brought about 

 the discovery of vaccination. He took the matter formed 

 in cowpox and introduced this vaccine virus (poison from 

 cows) into the blood of man. The germs from cowpox are 

 so much weaker than those that come from smallpox that 

 they cannot produce the latter disease; but they cause 

 the body to produce the substance which kills the small- 

 pox germ. The length of time for which vaccination 

 renders one immune varies with the person vaccinated, 

 lasting usually from one year to ten or twelve years. 



When vaccination is attended by ill effects in other parts 

 of the body or with very serious inflammation at the point 

 of vaccination, we may be sure that the trouble may be 

 laid to carelessness. The opening of the skin must be 

 kept clean and free from other infection. 



Diseases Carried by Insects. A few diseases are 

 caused by one-celled animals or protozoa. The protozoa 

 do not cause disease by being carried directly from one 

 person to another, but must live for a while in the body 

 of some insect. The germs are sucked up in the blood 



