Yersinia pestis, a small gram-negative rod, is the causative agent of plague and can be found worldwide. Humans are generally accidental hosts with the natural animal reservoirs being rodents. Transmission between rodents is via fleas, however infection can also result by ingestion of contaminated animal tissues or through the respiratory route.

From the site of innoculation, Y.pestis bacteria are carried by the lymphatic system to regional lymph nodes. The bacteria are taken up by phagocytes, however are resistant to killing, so survive and sometimes multiply within these cells. The affected lymph noded experience haemorrhagic necrosis and the phagocyte-released bacteria have sysnthesised a new envelope antigen which make them more resistent to phagocyte killing. Extracellular bacteria can then be spread further troughout the body and cause additional haemorrhagic lesions.

Treatment. The danger of overwhelming septicemia requires that antibiotic treatment be started as soon as possible. The antibiotic of choice is streptomycin, but gentamycin and tetracycline can also be used.

Vaccines. A formalin killed vaccine can be used for those at high risk of infection.