Black Death: Infectious Nature and Transmission Mechanisms

The Black Death (1347–1351) was an infectious pandemic caused by the bacterium Yersinia pestis.

Transmission Cycle

1. Reservoir: Wild rodents (rats, ground squirrels, prairie dogs) maintain enzootic cycles.
2. Vector: Fleas (Xenopsylla cheopis) feed on infected rodents; bacteria multiply in flea gut, blocking proventriculus → "blocked flea" regurgitates infected blood into next host.
3. Human infection: Flea bite → bubonic plague (lymphadenitis/buboes). Direct contact with infected tissues/fluids → septicemic or pneumonic forms.
4. Pneumonic spread: Respiratory droplets from coughing pneumonic patients → primary pneumonic plague (highly lethal, person-to-person).

Clinical Forms

• Bubonic: 80–85%; swollen lymph nodes (groin, axilla, neck), fever, sepsis if untreated.
• Septicemic: Bacteremia without buboes; DIC, necrosis, high mortality.
• Pneumonic: Pneumonia + hemoptysis; rapid progression, airborne transmission.

Modern Context

Endemic in rodent populations globally (SW US, Africa, Asia). Treatable with aminoglycosides, fluoroquinolones, doxycycline if diagnosed early. Surveillance and rodent control limit outbreaks.