214 BACTERIA, YEASTS, AND MOLDS 



these three factors. If we know how the bacteria leave 

 the body of the patient, how they are distributed, and how 

 they enter the body of another, we are well equipped to 

 guard against them. 



I. The Means of Elimination from the Body 



A knowledge of the means by which the contagious 

 material leaves the body of the patient is of first impor- 

 tance in preventing the distribution of such material, 

 and should always be the first question asked. There 

 are several different methods. 



The parasites that produce certain diseases do not find 

 any direct means of being eliminated from the body, and 

 when this is the case the disease is not in any proper 

 sense contagious. Malaria is the best example of this 

 class of diseases, and yellow fever is probably a second. 

 Malaria, chills and fever, or fever and ague, are all the 

 same disease, and are produced by a microscopic parasite 

 living in the human blood. Growing there, it develops 

 poisonous secretions, and these acting upon the body give 

 rise to the symptom of chill followed by fever only too 

 well known in this disease. The parasite is a minute little 

 body (Fig. 69, i) which enters the blood corpuscle. Inside 

 this corpuscle it grows, and finally breaks up into many lit- 

 tle bodies, or spores. As soon as the spores are formed 

 the blood corpuscle breaks to pieces, setting the spores 

 free and at the same time liberating the secreted poisons. 

 These poisons cause the chill followed by fever well 

 known in malaria. The spores may then enter into other 

 blood corpuscles and go through the same history again 



