MODES OF INFECTION 229 



tion, are exceedingly infective. Consequently, the immediate 

 neighbourhood of a person attacked by them is dangerous to 

 susceptible people. On the other hand, a person suffering from a 

 water-borne malady may be approached with safety, the danger 

 lying in the source, the nutriment (water or food) whence the 

 disease was acquired, or in the solids or liquids he may excrete. 

 Some species of microbes which enter by the breath, for example, 

 those of tuberculosis, are comparatively large and heavy. They 

 fall to the ground, whence, the sputum in which they are contained, 

 having dried, they are swept up with particles of dust by currents 

 of air. They are, therefore, termed earth-borne. As compared to 

 the air-borne types they are in lesser multitudes in the air sur- 

 rounding a sufferer whose neighbourhood, therefore, especially 

 in well-ventilated spaces, is not so charged with infection. The 

 microbes of other earth-borne diseases, for example those of 

 tetanus, gain entrance through abrasions in the skin. 



380. Yet other species, for example those of malaria and 

 sleeping-sickness, are insect-borne ; that is, they are conveyed 

 from an infected to a susceptible person by blood-sucking insects. 

 Lastly, some species, for example those which cause the venereal 

 diseases, pass by direct contact from one person to another. 

 Rabies, which is one of these ' contagious ' maladies, is hardly 

 ever, if ever, communicated by one human being to another, but 

 is acquired through the bite of an infected individual belonging 

 to some lower type of mammal. 



381. The terms contagious, earth, air, water, and insect-borne 

 are convenient as indicating common modes of infection, but 

 they do not necessarily imply that each of the various diseases 

 infects in only one way. For instance, scarlet fever is usually 

 air-borne and tuberculosis earth-borne; but it is probable that 

 both are sometimes conveyed by milk. During the war in South 

 Africa, enteric fever seems to have been spread by dust storms, 

 or by flies which had been feeding on excreta. In England 

 also flies are common vehicles for infantile diarrhoea during the 

 summer months. 



382. The bodies of the higher animals consist mainly of 

 nutritive substances, and would be happy hunting-grounds for 

 microbic types if they possessed no means of defence. Though the 

 skin and food and air passages of the higher animals swarm with 

 unicellular organisms, these are absent from the tissues of a healthy 

 individual. The fact that the microbes of disease are often able 

 to break down the defence and enter the tissues, whereas the 



