L.E.W. (MD)
The causative agent is Pasteurella pestis, a short, plump, pleomorphic, gram negative bacillus with a tendency toward bipolar staining. The incubation period varies from 2 to 10 days (average 3). Fluid from a bubo (or sputum, in the pneumonic type) contains the organism, which can be identified by culture and guinea-pig inoculation.