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T he microbiological cause of the “Black Death” was unknown until 1890, when Alexandre Yersin independently interpreted the plague bacilli in Hong Kong. He observed that many rats were infected and died during that deadly epidemic period. While studying black death or the plague, he examined some dead rats and found the same bacilli in the lymph nodes of all the rats, and these bacilli were also found in the tissue of infected humans. The lymph nodes were swollen. In the infected person and the rats, he isolated the organism from the buboes (swollen parts). He cultivated the isolated organism and observed the same bacilli in both humans and rats. So he concluded that the infection is carried from rats to humans, and he also noticed that the infected animal has the bacilli in its tissues. The isolated organism was gram negative, non-motile, coccobacillus without spores, and a facultative anaerobe, and it infected humans via fleas, rodents, and small animals (zoonotic). It was first named Bacterium pestis, then Pasturella pestis, and later Yersinia pestis in his honour