Plague is caused by the bacteria Yersinia petis, which is transmitted from rodent to rodent by infected fleas. If bitten by an infected flea, a person usually develops a bubonic form of plague, which is characterised by a swelling of the draining lymph node. While bubonic plague is the commonest, there are also the septicaemic form and pneumonic form. Septicaemic plague occurs when infection spreads directly through the bloodstream, while the pneumonic form is the most virulent but least common. Plague also spreads from human to human by inhalation of aerosolised infective droplets, in the absence of flea or animal.
A Hong Kong outbreak of plague occurred in Tai Ping Shan of Sheung Wan District in 1894. In fact, the bubonic plague bacillus was isolated in Hong Kong at that time by Alexandre Yersin from France.