Plague is usually transmitted to humans through the bite of a flea from an infected rodent host, such as a rat or prairie dog. Occasionally, transmission occurs from handling infected animals or their tissue; the infection is rarely transmitted from person to person. Respiratory droplets (coughing) are highly contagious in pneumonic plague, and transmission can occur after inhalation of Y. pestis in a laboratory.

Complications from plague include acute respiratory distress syndrome, chronic lymphedema from lymphatic scarring, disseminated intravascular coagulation, septic shock, and secondary infection of the buboes by Staphylococcus and Pseudomonas species.

In bubonic plague, the incubation period is 2 to 8 days. The milder form begins with malaise and fever in addition to pain, swelling, and tenderness in regional lymph nodes. Lymph node damage (usually axillary or inguinal) produces painful, inflamed, and possibly suppurative buboes. Hemorrhagic areas become necrotic in the skin and appear dark (“black death”). This infection can progress extremely rapidly within hours. Bubonic plague may also begin dramatically, with a sudden high fever of 103° to 106° F (39.4° to 41.1° C), chills, myalgia, headache, prostration, restlessness, disorientation, delirium, toxemia, and a staggering gait. Occasionally, abdominal pain, nausea, vomiting, and constipation followed by diarrhea (frequently bloody), skin mottling, petechiae, and circulatory collapse occur.

In pneumonic plague, the incubation period of 2 to 3 days is followed by high fever, chills, severe headache, tachycardia, tachypnea, dyspnea, and a productive cough (mucoid sputum at first, later frothy pink or red sputum). Pneumonic plague can quickly cause severe prostration, respiratory distress and, usually, death.

Septicemic plague usually develops without overt lymph node enlargement. The patient shows toxicity, hyperpyrexia, seizures, prostration, shock, and disseminated intravascular coagulation (DIC). Septicemic plague causes widespread nonspecific tissue damage (peritoneal or pleural effusions, pericarditis, and meningitis). With meningeal plague the patient has a fever, headache, nuchal rigidity, and buboes, especially axillary. Pharyngeal plague results from ingestion of the plague bacilli and produces symptoms of sore throat, fever, painful cervical lymph nodes, and pharyngeal erythema.