Rats were the reservoir hosts of the Y. pestis bacteria and the Oriental rat flea was the primary vector .
The plague seems to have been spread by flea - infected rats , as well as individuals who had been infected on the continent .
Rats were the rats   of the Y. pestis bacteria and the Oriental rat flea was the primary vector .
The plague seems to have been spread by flea - infected rats , as well as individuals who had been infected on the continent .