PATHS AND MODE OF INFECTION 433 



the system by the skin surface through small wounds, cracks, 

 abrasions, etc., and in such cases there is usually no reaction 

 at the site of entrance. This last fact is in accordance with 

 what has been stated above with regard to experiments on 

 monkeys. The path of infection is shown by the primary 

 buboes, which are usually in the glands through which the 

 skin is drained, those in the groin being the commonest site. 

 Absolute proof of the possibility of infection by the skin is 

 supplied by several cases in which the disease has been acquired 

 at post-mortem examinations, the lesions of the skin surface 

 being in the majority of these of trifling nature ; in only two 

 was there local reaction at the site of inoculation. In most of 

 these cases the period of incubation has been from two to three 

 days ; under natural conditions of infection the average period 

 is within five days. While infection may occur by accidental 

 inoculation through small wounds of the skin surface, it appears 

 in the majority of cases to take place by means of the bites 

 of fleas. For some time it had been known that plague bacilli 

 might be found for some time afterwards in the stomach of fleas 

 allowed to feed on animals suffering from plague, and some 

 observers, for example Simond, had succeeded in transmitting 

 the disease to other animals by means of the infected insects. 

 Most observers, however, had obtained negative results, and it 

 was only by the work of the Advisory Committee appointed by 

 the Secretary of State for India in 1 905 l that the importance 

 of this means of infection was established. By carefully planned 

 experiments the Committee showed that the disease could be 

 transmitted from a plague rat to a healthy rat kept in adjacent 

 cages when fleas were present ; whereas this did not occur when 

 means were taken to prevent the access of fleas, though the 

 facilities for aerial infection were the same. The disease can 

 also be produced by fleas removed from plague rats and trans- 

 ferred directly to healthy animals, success having been obtained 

 in fully 50 per cent of experiments of this kind. When plague- 

 infected guinea-pigs are placed amongst healthy guinea-pigs 

 comparatively few of the latter acquire the disease when fleas 

 are absent or scanty ; whereas all of them may die of plague 

 when fleas are numerous. This result demonstrates the com- 

 paratively small part played by direct contact, even when of 

 a close character. Important results were also obtained with 

 regard to the mode of infection in houses where there had been 

 cases of plague. It was found possible to produce the disease 

 in susceptible animals by means of fleas taken from rats in 



1 See Journal of Hygiene, vi. 421 ; vii. 323. 

 28 * 



