Yersinia pestis (Y. pestis; formerly Pasteurella pestis) is a gram-negative, non-motile, coccobacillus bacterium without spores that is related to both Yersinia pseudotuberculosis and Yersinia enterocolitica. It is a facultative anaerobic organism that can infect humans via the Oriental rat flea (Xenopsylla cheopis). It causes the disease plague, which caused the first plague pandemic and the Black Death, the deadliest pandemic in recorded history. Plague takes three main forms: pneumonic, septicemic, and bubonic. Yersinia pestis is a parasite of its host, the rat flea, which is also a parasite of rats, hence Y. pestis is a hyperparasite. Y. pestis was discovered in 1894 by Alexandre Yersin, a Swiss/French physician and bacteriologist from the Pasteur Institute, during an epidemic of the plague in Hong Kong. Yersin was a member of the Pasteur school of thought. Kitasato Shibasaburō, a Japanese bacteriologist who practised Koch's methodology, was also engaged at the time in finding the causative agent of the plague. However, Yersin actually linked plague with a bacillus, initially named Pasteurella pestis; it was renamed Yersinia pestis in 1944. Every year, between one thousand and two thousand cases of the plague are still reported to the World Health Organization. With proper antibiotic treatment, the prognosis for victims is much better than before antibiotics were developed. A five- to six-fold increase in cases occurred in Asia during the time of the Vietnam War, possibly due to the disruption of ecosystems and closer proximity between people and animals. The plague is now commonly found in sub-Saharan Africa and Madagascar, areas that now account for over 95% of reported cases. The plague also has a detrimental effect on non-human mammals; in the United States, these include the black-tailed prairie dog and the endangered black-footed ferret. General features Y. pestis is a non-motile coccobacillus, a facultative anaerobic bacterium with bipolar staining (giving it a safety pin appearance) that produces an antiphagocytic slime layer. Similar to other Yersinia species, it tests negative for urease, lactose fermentation, and indole. Its closest relatives are the gastrointestinal pathogen Yersinia pseudotuberculosis, and, more distantly, Yersinia enterocolitica. Genome and proteome Genome Several complete genome sequences are available for various strains and subspecies of Y. pestis: strain KIM (of biovar Y. p. medievalis), and strain CO92 (of biovar Y. p. orientalis, obtained from a clinical isolate in the United States). In 2006 the genome sequence of a strain of biovar Antiqua was completed. Some strains