238 BACTERIA, YEASTS, AND MOLDS 



sense contagious but produced by parasitic organisms which 

 may under peculiar conditions pass from individual to indi- 

 vidual. Prominent among this last class are malaria and 

 yellow fever, diseases 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, indeed, like measles and mumps, 

 while almost certainly caused by microorganisms of some 

 sort living in the human body, have not yet been satis- 

 factorily 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 likely 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. For a proper understand- 

 ing, therefore, of the way to handle contagious diseases 

 in the home we need to consider these three factors. 



