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How do plague bacteria travel through the human lymphatic system?

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two plague victims with sores lying in bed, while a robed figure gestures above them. — plague Image depicting victims of the sixth biblical plague.

When an infected flea bites a human, the Yersinia pestis bacteria are regurgitated into the host. From the site of the bite, the bacilli migrate through the lymphatic system to the lymph nodes.1

Once they reach the lymph nodes, the bacteria produce proteins that inhibit the body's normal inflammatory response and prevent them from being digested by macrophages, which are cells that typically fight infections. By weakening the host's immune defense, the bacilli are able to multiply rapidly within the lymph nodes. This colonization causes the nodes to become painfully swollen and tender, a characteristic sign of bubonic plague known as a bubo. If the infection is not treated, the bacteria may eventually destroy the lymph node tissue and potentially enter the bloodstream, leading to more severe forms of the disease.12