Photo: Rat Receiving Station set up as part of the War on Rats led by the U.S. Public Health Service. Millions of rats were killed and in 2 months no new cases of plague were reported.

Bubonic plague, or "the black death," had raged throughout Europe and Asia over the past centuries. In the twentieth century, it came to America.

In the summer of 1899, a ship sailing from Hong Kong to San Francisco had had two cases of plague on board. Because of this, although no passengers were ill when the ship reached San Franscisco, it was to be quarantined on Angel Island. When the boat was searched, 11 stowaways were found -- the next day two were missing. Their bodies were later found in the Bay, and autopsy showed they contained plague bacilli. Despite this scare, there was no immediate outbreak of disease. But rats from the ship probably had something to do with the epidemic that hit San Francisco nine months later.

On March 6, 1900, a city health officer autopsied a deceased Chinese man and found organisms in the body that looked like plague. In 1894, two research physicians had simultaneously and independently identified the bacillus that causes bubonic plague. Shibasaburo Kitasato published his findings in Japanese and English; Alexandre Yersin published in French. People in different parts of the world credited one or the other with the discovery, depending which journals they had read. (Since 1970 the bacillus has been known as Yersinia pestis.) That the plague had an identifiable "germ" was known. But other recent findings had not been disseminated -- or believed. Most people felt that the germ infected humans through food or open wounds. Disinfection campaigns were the order of the day. In some places they ran carbolic acid through sewers, actually spreading the disease faster because it flushed out rats that had lived there.

Back in San Francisco, however, political issues vied with scientific efforts. Anti-Chinese feeling ran strong in the city then, and the first step taken was to quarantine Chinatown. The Chinese objected, and so did the business community. Not because they wanted to protect the rights of the Chinese, but it was bad for business to have people thinking there was plague in their city or state. The quarantine was lifted but health officials ran house-to-house inspections of Chinatown. People resisted, hiding their dead and locking their doors. But two more plague victims turned up. The city Board of Health officially announced that plague was present in the city. The governor refused to believe it or to do anything to help in the antiplague effort. The Surgeon General got permission from President McKinley to pass antiplague regulations. Others still denied the existence of plague, although more and more states in the country were stopping trade with California.

Commissions and boards formed, fought with the governor, and were disbanded, underfunded, and reformed. Meanwhile, more plague cases were found. In April 1901, a clean-up campaign of Chinatown was undertaken, scouring almost 1,200 houses and 14,000 rooms. In 1903, a new governor took office and vowed to help the boards of health in every way. On February 29, 1904, a woman in the town of Concord, California, died of plague, its last victim -- for a while. There had been 121 cases in San Francisco and 5 outside, with 122 deaths.

Knowledge, like illness, spreads. In the next few years, information about the plague's causes and transmission would be clarified. In 1894, physician Mary Miles in Canton, China, had reported the widespread death of rats in plague epidemics. But people assumed the rats caught the disease from humans. In 1897, Japanese physician Masanori Ogata wrote "one should pay attention to insects like fleas for, as the rat becomes cold after death, they leave their host and may transmit the plague virus directly to man." Paul Louis Simmond put all the accumulated observations together, made his own observations (societal and microscopic), and experimented with the bacillus, rats, and fleas. He proved that rat fleas bit people (which went against received wisdom), and that a sick animal could not transmit the disease if it didn't have fleas. Simmond published his conclusions in 1898 and was roundly ridiculed. But in 1905, a British commission published some of the same findings and in 1908, issued a report confirming all of Simmond's conclusions (though not crediting him).

In 1906, an earthquake of record proportions devastated San Francisco. The ruin of the city's buildings made not just people, but rats, homeless. The subsequent year or two of living in refugee camps while rebuilding was highly conducive to rat and flea infestations. In 1907, cases of plague were reported. But with hindsight on the last epidemic and new knowledge from research, officials launched a new kind of campaign. They offered a bounty on rats. A similar rat-catching campaign had been used successfully to fight plague in New Orleans. It worked as well in San Francisco, and though this second epidemic was stronger than the first, it was brought to halt in 1909.