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332 future, more than they have been in the past, in devising schemes of quarantine and in attempts at stamping out the disease in already affected localities. The wholesale destruction of domestic vermin should go hand in hand with the isolation of plague-stricken patients.* The role of the flea in plague.— Yersin placed in the same cage healthy and plague-inoculated mice. The latter died first; but later the originally healthy uninoculated mice also succumbed proving that plague is communicable either through the atmosphere, by contact, or by ectozoa.

Yersin's experiment has been successfully repeated again and again, on mice, rats, guineapigs, and monkeys, and with many modifications. The result has been indisputable confirmation of Yersin's results, and further proof that bubonic plague is not communicable from animal to animal by simple confact or by atmospheric convection, but that it is readily communicated by ectozoa, especially rat-fleas —principally Xenopsylla cheopis † (Fig. 67) which act as passive intermediaries and carriers of the bacillus. Zirolia and others have found that Bacillus pestis multiplies in the stomach of the flea, retaining its virulence for over twenty days and being passed