Pneumonic plague. Chest X-ray of a patient with a progressive plague infection (cloudy areas) affecting both lungs. Plague is caused by the bacteria Yersinia pestis, which is spread to humans by bites from infected fleas carried on rats. Pneumonic plague is usually secondary to the infection caused by the flea bite, but may also be contracted directly, through inhalation, from another person with pneumonic plague. Infection is rapid, causing shortness of breath, chest pain, cough, and haemoptysis (coughing up blood), and can be fatal within a day. Prompt treatment with antibiotics can effect a cure.