METHODS AND CHANNELS OF INFECTION. 527 



various suctorial insects also may suck up the blood of one individual 

 and carry the infectious agent to the normal individual. Notable ex- 

 amples of this are found in the transmission of the various trypano- 

 somiases by the tsetse and other tropical flies, of Rocky Mountain 

 spotted fever by the wood tick. The same is true of Bad. pestis, of 

 plague, which is carried by the flea, of Texas fever by the cattle tick, 

 and it has been shown recently that the louse may be one of the agents 

 in the transmission of typhus fever. Under the third, the serving as an 

 intermediate host and the carrying of the causal agent, may be men- 

 tioned the mosquitoes which serve as the only means of transmission 

 of the causal microorganisms of malaria and yellow fever and in which 

 these parasites pass a certain cycle of their existence. 



HUMAN CARRIERS OF INFECTION. It has been previously mentioned 

 that man is capable of carrying infectious agents when he himself is not 

 infected. For example, in the case of diphtheria it has been repeatedly 

 shown that convalescents from diphtheria, persons who have had the 

 disease, and persons, who have never had the disease, frequently carry the 

 causal microorganisms of this disease in a virulent form and are accord- 

 ingly disseminators. Not uncommonly persons who have had typhoid 

 fever carry large 'numbers of virulent B. typhosus in their bodies, partic- 

 ularly in the gall bladder, and disseminate them thus causing many in- 

 fections. They may be carried for many years, one case of eighteen 

 years being on record. Possibly Asiatic cholera is also occasionally 

 carried for a period of time. Individuals who carry infectious organ- 

 isms are popularly known as "germ-carriers" and should be kept 

 under constant observation. 



CONTACT INFECTION. It is only necessary to emphasize certain 

 points in addition to what has been said in the foregoing. It has been 

 stated that animals may communicate an infectious agent to other animals 

 of the same or different susceptible species by direct contact. Prob- 

 ably the commonest diseases to be communicated by animals to each 

 other are tuberculosis and glanders. This is commonly accomplished 

 by the rubbing of the mouths and noses together, although the disease 

 may be acquired in other ways. Among the human race the diseases 

 usually communicated by the contact of one individual with another 

 are diphtheria, scarlet fever, smallpox, mumps, measles, gonorrhoea, 

 chancroid and syphilis. In these five first-mentioned diseases it seems 

 that the expirations carry the causal microorganism and that it is 



