68 THE RELATION OF BACTERIA TO DISEASE 



Scales from the skin in the acute eruptive diseases of 

 children may transmit infection. Milk and water 

 have been known to transmit diphtheria, typhoid, 

 scarlatina, and other conditions. Insects transmit 

 disease in two ways, mechanically and specifically. 

 Diseases like typhoid and tuberculosis may be trans- 

 mitted by flies, which soil themselves on excreta or 

 sputum and deposit the infective matter upon food 

 or other objects, which later get into the human body. 

 Other diseases probably to be credited in this category 

 are plague and diphtheria. 



In the other class of insect-born disease the trans- 

 mission can only take place by this means. Thus 

 malaria is only transmitted from the sick to the unin- 

 fected by the Anopheles mosquito, sleeping sickness 

 only by the tsetse fly, and yellow fever only by the 

 Stegomyia mosquito. In these insects there is a 

 development of the virus to such a degree that it can 

 be infective for an unprotected person, and for each 

 disease this so-called cycle of development is necessary 

 for its further propagation. None of the diseases 

 demanding an insect for its spread can be transmitted 

 by one person to another by the most intimate per- 

 sonal contact. It may be laid down as a law that with 

 the exception of the few infectious disorders only carried 

 by insects, intimate personal contact is the most pro- 

 lific source of the spread of disease. 



The objects before mentioned clothing, dishes, 

 books, utensils, and so forth called "fomites," were 

 formerly believed of considerable importance in trans- 

 mitting disease, but latterly more weight has been 

 laid upon individuals as carriers of viruses. This has 



