Fleas that ingest bacteria get bacterial growth in gut, blocks food flow. Fleas become ravenously hungry, go on feeding frenzy, repeatedly biting victim, spreading disease. Otherwise only by flea bites. But never likely to approach epidemic status since we know how it spreads, how to treat cases as they occur. Bacterium has waxy coat, can survive long periods in air and house dust.
Bacteria grow and multiply extremely slowly, inhibition takes week Antibiotic resistance .
Prolonged use of one drug is optimal situation to select for resistant strain. Once resistant strain becomes widespread, that antibiotic no longer effective. Humans are accidental host. Colonizes newborn infants within hours of birth. A classic opportunist .