How did port cities contribute to the worldwide spread of plague in the late 19th century?

In the late 19th century, major port cities—most notably Guangzhou and Hong Kong—functioned as critical distribution centers that facilitated the worldwide spread of the third plague pandemic.1

The disease, which gained momentum in China during the 1850s, utilized global maritime trade routes to reach distant regions. Because the mechanism of transmission was not yet fully understood, infected rodents carrying rat fleas traveled aboard seagoing vessels, allowing the pathogen to move rapidly between international ports. Between 1894 and 1922, this process introduced the plague to numerous major hubs, including Bombay, Cape Town, San Francisco, and Pensacola, ultimately resulting in more than 10 million deaths globally.123

This pandemic was only curtailed after scientists identified the bacterium Yersinia pestis and the role of fleas in its transmission, which prompted the implementation of worldwide rat-proofing measures in maritime vessels and port facilities.12