Only fleas which have been exposed to rats or other animals infected with the plague bacteria
pestis) can transmit it to humans.
Y. pestis colonizes the flea's gut, replicates, and blocks the flea's gut. The fleas then feed voraciously, regurgitating the bacteria into the bite. Only a few organisms are required to pass on the infection. Most cases have been in in CA, AZ, NM, & CO.