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