CONDITIONS OF CONTAGION 213 



sense contagious but produced by parasitic organisms 

 which may under peculiar conditions pass from individ- 

 ual to individual. Most prominent among this last class 

 is malaria, a disease never known to pass directly from 

 one person to another but which may be distributed from 

 individual to individual through an agency to be noticed 

 presently. It must not be assumed that science at the 

 present time knows the cause of all the diseases here 

 listed. Some of them, like measles, scarlet fever, whoop- 

 ing cough, and mumps, while almost certainly caused 

 by microorganisms living in the human body, have not 

 yet been satisfactorily explained, and we do not know 

 the actual germs which cause them. There are some 

 other contagious diseases besides those mentioned, for 

 almost any trouble that produces open sores anywhere 

 on or in the body is liable to be distributed from person 

 to person. Those mentioned are, however, the most 

 important. 



Conditions of Contagion 



To make it possible for a disease to pass from one person 

 to another, three conditions must be fulfilled: (i) The 

 microorganisms which produce the disease must find some 

 means of exit from the patient. (2) The organisms must in 

 some way be carried from the patient to the healthy indi- 

 vidual. (3) The organisms must find some means of enter- 

 ing the body of the healthy individual. If the parasites 

 can meet these three conditions, the disease will be carried 

 from patient to well person, and thus will be contagious. 

 For a proper understanding, therefore, of the way to han- 

 dle contagious diseases in the home we need to consider 



