The experiments above described have confirmed the observations of Laigret and Jadin that the European human typhus virus cannot be maintained for more than two generations in mice by brain-peritoneum passage; whereas the murine Mexican variety can be carried on by this method in mice through at least eleven passage generations. The fact that within eleven passages there is no attenuation of the murine virus renders it likely that this agent can continue in mice, in an inapparent form, without material modification.

Brill's disease virus from three different isolations has behaved like the European type, a fact which strengthens the opinion previously expressed from this laboratory that Brill's disease represents an imported European strain of the classical European infection.

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