Yersinia pestis In humans, Y. pestis can take three main forms, most notorious is bubonic, pneumonic, and septicemic plagues.  Bubonic Y. pestis infection is caused by flea vectors, and the bacteria enter through bites.  This causes swollen lymph nodes in the armpits, neck and groin.  Also, it is classified by red marks on the skin that turn black.  Pneumonic and septicemic plagues follow the bubonic plague if the patient gets pneumonia or blood poisoning.  Septicemic plague occurs if the patient gets blood poisoning, meaning the bacteria has entered the blood stream.  Pneumonic plague can occur two ways.  One, being that bubonic plague has infected the person, and they catch pneumonia.  The second way is if septicemic plague enters the lungs.  In this form, the plague can be transmitted person to person and carries a high mortality rate.  Symptoms include fever, headache, and weakness, shortness of breath, chest pain and cough, often coughing up blood.  Although not commonly seen in the industrialized world, patients that do get the plague generally fully recover with the help of antibiotics.