Like the two previous outbreaks, this one also originated in Eastern Asia, most likely in Yunnan, a province of China, where there are several natural plague foci. The initial outbreaks occurred in the second half of the 18th century. The disease remained localized in Southwest China for several years before spreading. In the city of Canton, beginning in January 1894, the disease had killed 80,000 people by June. Daily water-traffic with the nearby city of Hong Kong rapidly spread the plague there, killing over 2,400 within two months during the 1894 Hong Kong plague.The third pandemic spread the disease to port cities throughout the world in the second half of the 19th century and the early 20th century via shipping routes. The plague infected people in Chinatown in San Francisco from 1900 to 1904, and in the nearby locales of Oakland and the East Bay again from 1907 to 1909. During the former outbreak, in 1902, authorities made permanent the Chinese Exclusion Act, a law originally signed into existence by President Chester A. Arthur in 1882. The Act was supposed to last for 10 years, but was renewed in 1892 with the Geary Act, then followed by the 1902 decision. The last major outbreak in the United States occurred in Los Angeles in 1924, though the disease is still present in wild rodents and can be passed to humans that come in contact with them. According to the World Health Organization, the pandemic was considered active until 1959, when worldwide casualties dropped to 200 per year.
Bubonic plague is one of three types of plague caused by the plague bacterium (Yersinia pestis). One to seven days after exposure to the bacteria, flu-like symptoms develop. These symptoms include fever, headaches, and vomiting, as well as swollen and painful lymph nodes occurring in the area closest to where the bacteria entered the skin. Acral necrosis, the dark discoloration of skin, is another symptom. Occasionally, swollen lymph nodes, known as "buboes," may break open.The three types of plague are the result of the route of infection: bubonic plague, septicemic plague, and pneumonic plague. Bubonic plague is mainly spread by infected fleas from small animals. It may also result from exposure to the body fluids from a dead plague-infected animal. Mammals such as rabbits, hares, and some cat species are susceptible to bubonic plague, and typically die upon contraction. In the bubonic form of plague, the bacteria enter through the skin through a flea bite and travel via the lymphatic vessels to a lymph node, causing it to swell. Diagnosis is made by finding the bacteria in the blood, sputum, or fluid from lymph nodes.Prevention is through public health measures such as not handling dead animals in areas where plague is common. While vaccines against the plague have been developed, the World Health Organization recommends that only high-risk groups, such as certain laboratory personnel and health care workers, get inoculated. Several antibiotics are effective for treatment, including streptomycin, gentamicin, and doxycycline.Without treatment, plague results in the death of 30% to 90% of those infected.