In 1878, not long after Reconstruction ended, Memphis appeared likely to emerge from the ashes of Confederate defeat as one of the regal cities of the New South. Her population had doubled during the 1860s in spite of war and occupation, and by 1878 it had reached nearly 48,000. She lay almost midway between New Orleans and St. Louis and had rail and river connections to all the major cities and growing markets of the South. Her experienced merchants, bankers, and warehousemen were ready to collect and sell cotton and other commodities produced in the rich hinterlands of western Tennessee and northern Mississippi and Alabama.

Moreover, she had something special for a resolutely Southern city sitting on the Chickasaw bluffs—a touch of cosmopolitanism. Of the 25,000 white Memphians in 1870, some 7,000 were foreign-born. The biggest group was Irish, and the next German, but there were sprinklings of Italians and French, a few Chinese residents, and enough Jews to sustain a Hebrew Benevolent Society.

But in 1878 the future of Memphis was broken and reshaped by ten weeks of epidemic. An existing community faded into memoir and album, and a new urban society appeared—one with lowered expectations, a different population mix, and a new political stance, part Progressive and part Old South. This overnight transformation was brought by a plague of mosquitoes.

THE STORY OFFENDS our modern sense of how history works. We like to believe that social and economic creations hatch slowly and comprehensibly. True, we know that our cities of today can vanish overnight if war comes, but the threat is of man-made storms of rage. Our late-twentieth-century view does not encompass what the Old Testament calls the “pestilence that walketh in darkness.” Yet it struck in America in 1878, when our great-grandparents were alive, when the telephone and the internal-combustion engine were already invented and the germ theory of disease had been formulated.

We can easily forget how far we have come, and how fast: as late as 1918 a worldwide influenza epidemic took over 20,000,000 lives. But even if we do remind ourselves of the links between disease and history, we are light-years from knowing emotionally what it means to live in an epidemic-stricken community where two or three hundred of our neighbors—of all ages and both sexes—die every day. Few of us can imagine the feeling that if we awaken with a headache or a chill, we may have only a week to live. A citizen of Boccaccio’s fourteenth-century Florence, smitten with the Black Death, could better understand the ordeal of Memphians in 1878 than could we.

The yellow fever outbreak that hit Memphis in 1878 came north up the river from New Orleans. Before it had run its course, it also ravaged other cities, big and small, in the Mississippi, Ohio, and Tennessee river valleys. Two hundred places in all, and 20,000 dead. But Memphis was hit most savagely.

Yellow fever was no stranger to America. It was, in fact, an old story to any trading town that trafficked with the Caribbean area, where it was rife. It had hit colonial Boston and New York. It had invaded Philadelphia—the capital of the United States—in 1793. And as new American ports rose in the South, it introduced itself there too. Norfolk, Charleston, Mobile, and Galveston suffered recurrent visitations. And New Orleans especially, with four epidemics in the 1850s leaving a total of 20,000 dead. Memphis got its first serious taste (1,250 cases, 220 deaths) in 1855.

Nineteenth-century Americans were baffled by the mystery of how yellow fever got around, appearing and vanishing without any discernible pattern. Here is what they did not know: The key factor was a small, silvery mosquito, nowadays called Aëdes aegypti (formerly Stegomyia fasciata ), which has since been eradicated from the Americas. The pregnant female of the species feeds on the blood of mammals, especially humans. She lays her eggs in clear, standing water, like that in open jars, pots, and wells found in and around human habitations. She is, in short, a sociable parasite.

And fatal. If she bites someone already in the early stages of yellow fever, she ingests the virus that causes the disease, and it multiplies by the millions in her tissues. It does not harm her, but after two weeks her bite will transfer it to any human being, who may in turn pass it along to other feeding mosquitoes, and so on in a vicious chain reaction. Since one mosquito can lay three to ten dozen eggs that become adult mosquitoes in less than ten days, the air can soon fill with thousands of them. They have a short flight range, but if the human population is dense enough and at least one person has yellow fever, any number of bugs can pick it up and infect several victims daily before getting a full belly.