cells to the area of infection. y. pestis is a versatile killer. in addition to rodents and humans, it is known to have killed camels, chickens, and pigs. domestic dogs and cats are susceptible to plague, as well, but cats are more likely to develop illness when infected. in either, the symptoms are similar to those experienced by humans, and can be deadly to the animal. people can be exposed by coming into contact with an infected animal (dead or alive), or inhaling infectious droplets that a sick dog or cat has coughed into the air. immunity main article: plague vaccine a formalin-inactivated vaccine was available in the united states for adults in 1993 at high risk of contracting the plague until removal from the market by the food and drug administration. it was of limited effectiveness and could cause severe inflammation. experiments with genetic engineering of a vaccine based on f1 and v antigens are underway and show promise. however, bacteria lacking antigen f1 are still virulent, and the v antigens are sufficiently variable such that vaccines composed of these antigens may not be fully protective. the united states army medical research institute of infectious diseases has found that an experimental f1/v antigen-based vaccine protects crab-eating macaques, but fails to protect african green monkey species. a systematic review by the cochrane collaboration found no studies of sufficient quality to make any statement on the efficacy of the vaccine. isolation and identification y. pestis isolated by ricardo jorge during the 1899 porto plague outbreak in 1894, two bacteriologists, alexandre yersin of switzerland and kitasato shibasaburō of japan, independently isolated in hong kong the bacterium responsible for the 1894 hong kong plague. though both investigators reported their findings, a series of confusing and contradictory statements by kitasato eventually led to the acceptance of yersin as the primary discoverer of the organism. yersin named it pasteurella pestis in honor of the pasteur institute, where he worked. in 1967, it was moved to a new genus and renamed yersinia pestis in his honor. yersin also noted that rats were affected by plague not only during plague epidemics, but also often preceding such epidemics in humans and that plague was regarded by many locals as a disease of rats; villagers in china and india asserted that when large numbers of rats were found dead, plague outbreaks soon followed. in 1898, french scientist paul-louis simond (who had also come to china to battle the third pandemic) discovered the rat–flea vector that drives the disease. he had noted that persons who became ill did not have to be in close contact with each other to acquire the disease. in yunnan, china, inhabitants would flee from their homes as soon as they saw dead rats, and on the island of formosa (taiwan), residents considered the handling of dead rats heightened the risks of developing plague. these observations led him to suspect that the flea might be an intermediary factor in the transmission of plague, since people acquired plague only if they were in contact with rats that had died less than 24 hours before. in a now classic experiment, simond demonstrated how a healthy rat died of the plague after infected fleas had jumped to it from a rat that had recently died of the plague. the outbreak spread to chinatown, san francisco, from 1900 to 1904 and then to oakland and the east bay from 1907 to 1909. it has been present in the rodents of western north america ever since, as fear of the consequences of the outbreak on trade caused authorities to hide the dead of the chinatown residents long enough for the disease to be passed to widespread species of native rodents in outlying areas. three main strains are recognised: y. p. antiqua, which caused a plague pandemic in the sixth century; y. p. medievalis, which caused the black death and subsequent epidemics during the second pandemic wave; and y. p. orientalis, which is responsible for current plague outbreaks. 21st century on january 15, 2018, researchers at the university of oslo and the university of ferrara suggested that humans and their parasites (most likely fleas and lice at the time) were the biggest carriers of the plague. ancient dna evidence in 2010, researchers in germany definitively established, using pcr evidence from samples obtained from black death victims, that y. pestis was the cause of the medieval black death. in 2011, the first genome of y. pestis isolated from black death victims was published, and concluded that this medieval strain was ancestral to most modern forms of y. pestis. in 2015, cell published results from a study of ancient graves. plasmids of y. pestis were detected in archaeological samples of the teeth of seven bronze age individuals, in the afanasievo culture in siberia, the corded ware culture in estonia, the sintashta culture in russia, the unetice culture in poland, and the andronovo culture in siberia. in 2018, the emergence and spread of the pathogen during the neolithic decline (as far back as 6,000 years ago) was published. a site in sweden was the source of the dna evidence and trade networks were proposed as the likely avenue of spread rather than migrations of populations. there is evidence that suggests y. pestis may have originated in europe in the cucuteni–trypillia culture, not in asia as is more commonly believed. dna evidence published in 2015 indicates y. pestis infected humans 5,000 years ago in bronze age eurasia, but genetic changes that made it highly virulent did not occur until about 4,000 years ago. the highly virulent version capable of transmission by fleas through rodents, humans, and other mammals was found in two individuals associated with the srubnaya culture from the samara region in russia from around 3,800 years ago and an iron age individual from kapan, armenia, from around 2,900 years ago. this indicates that at least two lineages of y. pestis were circulating during the bronze age in eurasia. the y. pestis bacterium has a relatively large number of nonfunctioning genes and three "ungainly" plasmids, suggesting an origin less than 20,000 years ago. on september 8, 2016, the y. pestis bacterium was identified from dna in teeth found at a crossrail building site in london. the human remains were found to be victims of the great plague of london, which lasted from 1665 to 1666. in 2021, researchers found a 5,000-year-old victim of y. pestis, the world's oldest-known, in hunter-gatherer remains in the modern latvian and estonian border area. events 496 were reported in the united states between 1970 and 2020. cases have been found predominantly in new mexico, arizona, colorado, california, oregon, and nevada. in 2008, plague was commonly found in sub-saharan africa and madagascar, areas that accounted for over 95% of the reported cases. in september 2009, the death of malcolm casadaban, a molecular genetics professor at the university of chicago, was linked to his work on a weakened laboratory strain of y. pestis. hemochromatosis was hypothesised to be a predisposing factor in casadaban's death from this attenuated strain used for research. on november 3, 2019, two cases of pneumonic plague were diagnosed at a hospital in beijing's chaoyang district, prompting fears of an outbreak. the patient was a middle-aged man with fever, who had complained of difficulty breathing for some ten days, accompanied by his wife with similar symptoms. police quarantined the emergency room at the hospital and controls were placed on chinese news aggregators. on 18 november a third case was reported, in a 55-year-old man from xilingol league, one of the twelve mongolian autonomous regions in northern china. the patient received treatment, and 28 symptomless contacts were placed in quarantine. in july 2020, officials increased precautions after a case of bubonic plague was confirmed in bayannur, a city in china's inner mongolia autonomous region. the patient was quarantined and treated. according to china's global times, a second suspected case was also investigated, and a level 3 alert was issued, in effect until the end of the year. it forbade hunting and eating of animals that could carry plague, and called on the public to report suspected cases.