The latest PBS American Experience episode is “Plague at the Golden Gate,” about the first known bubonic plague outbreak in the United States. My nonfiction book for young adults, Bubonic Panic: When Plague Invaded America, covers the same ground. It includes many of the same photographs as well as new ones.

Both Bubonic Panic and the film documentary show American public health officials struggling to cope as plague strikes San Francisco in March 1900. The event has similarities to today’s ongoing COVID pandemic:  Transmission mysteries. Travel bans. Vaccine hesitancy. Politics. Economic fallout. Misinformation. Lessons from the world’s third plague pandemic apply to our current pandemic.

Bubonic Panic  tells an additional part of this story.

In 1894, Western doctors realized that bubonic plague was beginning to spread in China. Bacteriologists rushed to the British colony of Hong Kong in June 1894, hoping to identify the bacterium behind plague. Once they had, they believed they could develop treatments and vaccines to fight it. 

Alexandre Yersin was a Swiss-born bacteriologist who trained at the Pasteur Institute in France. Working in Hong Kong in 1894, he isolated the bacterium that caused plague. Eventually it was named Yersinia pestis in his honor. Yersin had heard the old superstition claiming that a great rat die-off was the omen of a human plague epidemic. He noticed dead rats everywhere in Hong Kong, and he wondered if the superstition might be true. 

To find out, Yersin collected rat corpses and took samples of their lymph nodes and blood. He examined the fluid under the microscope. He saw plague bacteria! When he injected mice with the bacteria, they died. Yersin had made an important breakthrough, proving that rats were a major vector in spreading plague.

Three years later, Japanese scientist Masanori Ogata examined fleas on a plague-infected rat in Taiwan and found plague bacteria mixed with the insect’s previous blood meal. Ogata suggested that fleas in search of a host might transmit plague bacteria by jumping off a dead rat onto a warm body that passed by . . . another rat or a human. 

The next year, in 1898, French bacteriologist Paul-Louis Simond, proved this hypothesis. After observing the pattern of outbreaks in Bombay (now Mumbai), India, he conducted an elegant experiment showing that fleas from plague-infected rats carried the bacteria from rat to rat and from rat to human by biting them.

But despite the insightful work of the three scientists, almost nobody believed them. In 1899, the Indian Plague Commission—a group of British and Indian researchers—discounted these experiments, doubting that rat fleas transmitted plague. Following the Commission’s advice, public health authorities around the world thought they could control plague the same way they handled other contagious diseases: Isolate the sick. Quarantine those exposed. Sanitize the homes and belongings of plague victims with disinfectant chemicals or fire. Blockade neighborhoods.

These measures failed to keep infected rats and fleas away from people. By the dawn of the twentieth century, plague had reached thirty countries on five continents and had taken the lives of tens of thousands of people. Then on March 6, 1900, the first American plague death was confirmed in San Francisco. Unaware of the flea experiments, U.S. public health officials attacked the problem with disinfection, quarantines, and blockades.

It took nearly ten years for the “rat–flea–human” research of Yersin, Ogata, and Simond to be accepted by the medical and scientific communities. The delay cost millions of people their lives.

In San Francisco, public health leaders finally realized that the way to control plague was to control rats and the fleas they carried. Their new approach: Deprive the rodents of food and places to hide. Trap and kill as many as possible. After two waves of plague outbreaks over a nine-year period, the disease was brought under control in the city.

The rest of the story?
By 1903, plague had already escaped San Francisco and entered the ground squirrel population in areas beyond the city. Soon the disease spread even farther. Today, plague bacteria infect about 75 species of mammals in 17 western states. On average in America, 7 people a year develop the disease.