Yersinia pestis (formerly Pasteurella pestis) is a Gram-negative, nonmotile, rod-shaped, coccobacillus bacterium, with no spores. It is a facultative anaerobic organism that can infect humans via the Oriental rat flea. It causes the disease plague, which takes three main forms: pneumonic, septicemic, and bubonic plagues. All three forms were responsible for a number of high-mortality epidemics throughout human history, including the sixth century's Plague of Justinian; the Black Death, which accounted for the death of at least one-third of the European population between 1347 and 1353; the Great Plague of London of 1665, which was ended in 1666 by the Great Fire of London; and the Third Pandemic, sometimes referred to as the Modern Plague, which began in the late 19th century in China and spread by rats on steamboats, claiming close to 10,000,000 lives. These plagues likely originated in China and were transmitted west via trade routes. Recent research in 2018 indicated that the pathogen may have been the cause of what was described as the Neolithic Decline, in which European populations declined significantly. This would push the date to much earlier and might be indicative of an origin in Europe rather than Eurasia.