The Y. pestis bacteria lives in the digestive tract of fleas. When an infected flea bites a person, it deposits the bacteria into the skin. Some of these bacteria stay in the skin, where they are attacked by neutrophils. Because this defence does not stop the spread of the infection, some scientists speculate that Y. pestis interferes with normal neutrophil function. The remaining bacteria leave the skin tissue, advancing the infection into the lymphatic system.

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