292 APPLIED BIOLOGY 



tainly due to animals which produce effects similar to those 

 of bacteria in other diseases. 



261. Infectious Diseases not yet Understood. The fol- 

 lowing diseases are almost certainly due to micro-organisms 

 which are readily transmitted from one person to another, 

 thus making them infectious; but the organisms have not 

 been discovered. The diseases of unknown causation are 

 yellow fever, hydrophobia, smallpox, whooping-cough, 

 measles, scarlet fever, and mumps. The last five of these 

 infectious diseases are highly contagious. In all these 

 diseases it has been possible for medical men to do much 

 towards controlling their spread by working on the assump- 

 tion that some undiscovered micro-organism is the cause, 

 and that therefore cases of the diseases should be handled 

 according to principles based on diseases known to be caused 

 by bacteria. For example, yellow fever has been shown 

 to be transmitted by bites of certain kinds of mosquitoes 

 which have previously sucked blood from a yellow-fever 

 patient ; and this has suggested the desirability of applying 

 the rules first worked out for malaria when it was discovered 

 that the cause is microscopic animals which are injected into 

 human blood by mosquito bites. The rules simply require 

 destruction of the mosquitoes ( 329), and preventing them 

 from biting healthy persons and especially those sick with 

 malaria or yellow fever. Similarly, in dealing with smallpox, 

 scarlet fever, mumps, measles, and whooping-cough, physi- 

 cians have assumed the existence of some undiscovered 

 micro-organisms, and have required isolation of patients, 

 quarantining of exposed persons, and disinfection of rooms 

 and of all articles on which bacteria from patients might 

 have lodged. 



262. How Bacteria cause Disease. We may take diph- 

 theria as an example in showing how bacteria cause disease. 

 We all know that the events connected with this dreaded 

 disease are as follows : A child is " exposed " to diphtheria, 



