Plague is a vector-borne illness, meaning it requires a living host to carry it from one animal to another. Most of the time, a specific species of flea -- Xenopsylla cheopis -- is the vector. Also known as the Oriental rat flea, Xenopsylla cheopsis prefers to feed on rats and other rodents, which can carry plague.
The Oriental rat flea has a physical trait that makes it very efficient at transmitting plague. Its digestive system can become blocked by a large mass of plague bacteria. When a blocked flea bites a host, it often regurgitates plague-infected blood back into the wound. Fleas that aren't prone to blockage, like the human flea, may still transmit plague by carrying bacteria on their mouthparts.
After the infected flea bites the host, the bacteria suppress the body's natural inflammatory response. They also use proteins to protect themselves from the immune system. For these reasons, it's not immediately obvious that anything is wrong.
The bacteria hitch a ride into the nearest lymph node, using white blood cells to carry them. Once the bacteria reach a lymph node, they multiply. Due to the overwhelming presence of bacteria and the endotoxins in their cell walls, the lymph node begins to swell. In a few days, the node becomes a painful, egg-sized bubo. The body's natural immune defenses kick in, causing a high fever in an attempt to kill the bacteria. Chills, muscle pain and weakness are also common.
If the infected flea bites the victim on the hand or arm, the bubo forms in the axillary lymph nodes under the arm. If it bites the foot or leg, the bubo forms in the inguinal lymph nodes in the groin. A bite to the head causes a bubo in the maxillary lymph nodes in the neck and jaw. If a flea bites the victim's torso, the bubo can form in the abdominal cavity, where doctors may not detect them.
Unless multiple plague-carrying fleas bite a person, bubonic plague generally causes only one bubo. Sometimes, it may cause a few buboes in the same cluster of lymph nodes. This is one of the reasons why some researchers doubt that bubonic plague was the culprit behind the Black Death and other pandemics. Some historical accounts describe victims as covered in buboes, which doesn't generally happen with bubonic plague.
Next, we'll look at two other forms of plague and how they differ from bubonic plague.