Yersinia pestis is the cause of the plague; it is also known as the “Black Death” due to the millions of victims it caused. There are three pandemics recorded. The first major event occurred in 541-767 AD, Central and East Africa were hit by plague, killing 40-100 million people.(1) The second major event occurred in 1346-1800 in parts of Central Asia and of Europe, killing approximately 25 million. The third major event occurred in 1800 in China, killing approximately 12 million people. pestis was discovered in 1800 by Alexander Yersin.(1,2) He studied this bacterium by extracting the microorganism from the lymph nodes of infected dead individuals and injecting it into live guinea pigs.(1) Later, in 1896, he developed a remedy to treat disease caused by Y. pestis and also discovered a relationship between rats and Y. pestis infection.(2) However, it was Paul-Louis Simond who actually discovered that it was the oriental rat flea that was the vector infecting rats and other mammals .(1,2)Plague is a zoonotic disease; prairie dogs and squirrels have been found to be commonly infected with Y. pestis; however, other larger and smaller mammals can also be infected.(1) Y. pestis can cause a variety of different symptoms and can manifest as bubonic, septicemic, or pneumonic forms. Due to the high infectious potential and severity of the organism, it has been used as a bioterrorism agent in the past. Etiology Y. pestis is part of the Enterobacteriaceae family. This microorganism is gram negative and grows in most environments as it is a facultative anaerobe. (1,2) Y. pestis is described as coccobacillus when viewed under a microscope and does not form spores. Y. pestis has a wide temperature range for growth; ...... half of the article ...... enucleated strains, such as the EV76 strain.(1,2,3) The EV76 live vaccine is commercially available and used worldwide.(3) Additionally , there is a 1 to 20% chance that this vaccine will cause mild side effects and an even lower chance that serious side effects will occur. (3) The factor that makes the vaccine effective is that that vaccine contains the F1 antigen to induce an immune response in the individual. (1,2,3) However, there have been strains lacking the F1 antigen that make the vaccine unusable . Therefore, researchers developed a vaccine that includes the F1 antigen and LcVR (V antigen).(3) LcVR is part of the type III secretion system in Y. pestis and the addition of this component can provide immunity against F1 negative strains. (3) Vaccine candidates for future studies may include the different types of Yops that Y. pestis can use for virulence.(2)