TOXINS, IMMUNITY, ETC. 485 



it, is of special importance in maintaining the disease from 

 season to season. The year may be divided into two portions 

 an epizootic season, from December to May inclusive, and a 

 non-epizootic, from June to November. During the latter 

 period there are few cases of plague in rats on account of fleas 

 being scanty; especially is this so in the case of mus rattus. 

 In fact, in certain villages where this species alone is present, 

 tin- disease may actually die out at the end of the epizootic 

 season, and accordingly when plague reappears in these places 

 this is due to a fresh importation a fact of great practical 

 i 1 1 1 { M rtance. A fresh epizootic first affects chiefly mus decumanus, 

 and a little later spreads to mus rattux, while a little later still 

 the disease attacks the human subject in the epidemic form; 

 in each case fleas form the vehicle of transmission, and an 

 interval of from ten to fourteen days intervenes between the 

 outbreak of the epizootic and that of the epidemic. The 

 proportion of cases of plague in mus decumanus is much higher 

 than in ///// rattus, for the reason mentioned. It has been 

 further shown that the bacilli flourish in the stomach of the 

 flea and are passed in a virulent condition in the faeces, that a 

 large proportion of fleas removed from plague-infected rats 

 contain plague bacilli, and that the fleas may remain infective 

 for a considerable nmnlxsr of days, sometimes for a fortnight. 

 The repeated contamination of flea-bites by means of the 

 excrement of fleas seems to be the most likely means of infection 

 of the human subject. 



hi primary plague pneumonia, from a consideration of the 

 anatomical changes and the clinical facts, the disease may be 

 said to be produced by the direct passage of the bacilli into the 

 respiratory passages. Nevertheless there must be certain factors, 

 still imperfectly understood, which determine the incidence of 

 this form : as in some epidemics of the highest virulence, plague 

 pneumonia has been practically absent, though opportunities for 

 infection by inhalation must have been present. On the other 

 hand, a case of plague pneumonia is of great infectivity in 

 producing other cases of plague pneumonia. If we except 

 infection through the respiratory passages in such cases, it may 

 In said that direct infection from patient to patient is relatively 

 uncommon. This is in accordance with the fact that in bubonic 

 plague the bacilli are not discharged from the unbroken surface 

 of the body, and are only present in the secretions in severe 

 cases. 



Toxins, Immunity, etc. As is the case with most organisms 

 which extensively invade the tissues, the toxins in plague 



