Plague is an acute bacterial infection caused by Gram negative organism Yersinia 
pestis. This bacteria is subdivided into three classical biotypes: Orientalis, 
Medievalis and Antiqua. Plague is transmitted via flea vectors from rodents to 
humans and by respiratory droplets from animals to humans or humans to humans. 
This agent is on the top of the CDC list of "Critical Biological 
Agents"--category A. It appears to be a good candidate agent for a bioterrorist 
attack. Type III secretion (TTS) is a mechanism by which Y. pestis communicates 
with eukaryotic cells by injecting bacterial proteins across cellular membranes 
into the cytosol of these cells. These bacterial proteins take control of the 
host cells by hijacking their intracellular machinery. A laboratory diagnostics 
of plague is based on: staining techniques, culture on media, 
immunochromatography, hemagglutination, immunofluorescence, ELISA, phage tests 
and genetical techniques including: PCR, multiplex and nested PCR, real time 
PCR, VNTR, PFGE, ISBF and Microarray.