Bettman / Getty

As COVID-19, the disease caused by the coronavirus, has spread in recent weeks, so has bad information. Widely shared “tips” have included gargling with salt water or vinegar (thought to eliminate the virus); holding your breath for 10 seconds to see whether you can do it without coughing or experiencing distress (if you can, you supposedly don’t have the virus); taking a few sips of water at least every 15 minutes (based on the theory that it will wash the virus into your stomach, where acid will kill it). None of these tips are backed by evidence. Still, despite early efforts by Facebook, Instagram, and Twitter to clamp down on false advice, the misinformation has continued to proliferate.

In the 18th century, it wasn’t just folk remedies that led people astray. During the yellow-fever epidemic of 1793—localized to Philadelphia, then the nation’s capital—doctors dispensed advice and “cures” that were sometimes quite harmful. From a 21st-century perspective, their desperate efforts would be laughable if the consequences hadn’t been so tragic, especially when the remedy, to modern eyes, was quite apparent. People looking back at the pandemic of 2020, centuries from now, may shake their head at something equally “obvious” that doctors are missing today.

Yellow fever wasn’t uncommon in the southeastern United States; Philadelphia’s summer outbreak was unusual not only for its northerly location but also for its virulence. Over the course of four months, the epidemic would claim the lives of 5,000 residents, about 10 percent of the city’s population.

At first, the city’s doctors were divided on whether the disease that was rapidly killing off residents was anything unusual, with some dismissing the illness as run-of-the-mill “autumn fever.” But Benjamin Rush, famous not only for being a physician but also for being a patriot during the Revolution, recognized the symptoms of yellowish skin and the vomiting of blood and black bile. He pronounced the sickness yellow fever, which hadn’t erupted in the city for 30 years.

Even after agreeing on the problem, doctors disagreed on the cause. Some, like Rush, attributed the disease to local conditions—specifically to a load of coffee from Santo Domingo rotting on a city wharf, which was thought to be exuding noxious fumes. Others suspected the recent influx of refugees fleeing a slave rebellion in present-day Haiti, many of whom spoke of fevers back home. Either way, as some are wont to do now, they blamed the disease on another country.

Although germ theory wouldn’t come along for another 70 years or so, people felt they could catch the disease by coming into contact with those who had it. More than 100 years later, scientists would discover that urban yellow fever is actually carried primarily by the mosquito Aedes aegypti, which has to bite an infected person and then transmit the disease by biting someone else. Philadelphians did notice an unusual number of mosquitoes in 1793, but failed to connect the insects to the disease.

Some medical recommendations issued in 1793 make sense now, while others seem absurd. Avoiding “all unnecessary intercourse” with infected persons and placing “a mark upon the door or window” of their house? Sure. Keeping the streets clean and avoiding “all fatigue of body or mind”? Okay. Strewing two inches of fresh earth in a room and changing the dirt twice daily, or taking frequent warm baths followed by ingesting five grains each of myrrh and black pepper? Not quite.

Doctors tried the traditional methods of coping with disease: alcoholic cordials, cool baths, bedclothes soaked in vinegar, and especially—to correct what was thought to be an imbalance in the body’s “humors”—drawing blood and “purging” with emetics and laxatives. Nothing worked. One commentator at the time recalled that Benjamin Franklin had observed, years before, that the inhabitants of Barbados began to recover from the fever only after the doctors had run out of medicine.

After a few weeks, Rush became convinced that the bleeding and purging simply hadn’t been aggressive enough. He began administering large amounts of mercury as a purgative—doses criticized by some of his colleagues as “murderous” and fit only for a horse. If the other doctors had also known that mercury was toxic even at lower doses, no doubt the criticism would have been even more severe. Rush also advocated drawing four-fifths of a patient’s blood. Because, like other doctors, Rush believed that the human body contained more than twice as much blood as it actually does, that amounted to far more blood than his already-weakened patients had in the first place. Many of his patients died, and some blamed Rush’s “cure.” But it wasn’t clear that other approaches were working any better, and somehow, enough people managed to recover to give Rush and many others confidence in his methods.

Given the ineffectiveness—or downright harmfulness—of most 18th-century medicine, that people turned to folk remedies is understandable. In an attempt to purify the supposedly noxious air, residents lit bonfires and shot off guns—a clear danger in a hot and crowded city, and discouraged by doctors. But the governor of Pennsylvania himself took things a step further, ordering a small cannon to be hauled through the streets, stopping every few yards to fire.

While today people in hot spots like New York City try to defy mandates to “shelter in place,” the “bad air” theory of disease led Philadelphia doctors to advise fleeing the area. Everyone who could afford to escape tried to do so, clogging roads and overburdening the modest inns that were often travelers’ only options. Before the end of the fever, nearly 20,000 had left. The many abandoned houses led to a problem we don’t see today, at least not yet: looting. On the other hand, the economic effects were similar. One resident who stayed in the city reported in September that “business of every kind is stopped, and provisions double price.”

People also started to fear Philadelphians. Major Christian Piercy, a Philadelphia potter, fell ill in a stagecoach in New Jersey. The other passengers forced him out, and a local landowner would allow him only an empty log cabin—where Piercy died alone, almost immediately. As far away as Massachusetts, Governor John Hancock issued a travel ban, directing each town to examine “all Persons, with the Baggage, and other Effects, by Land or Water, coming from Philadelphia, or any other infected Place.” Postmasters used tongs to dip letters from Philadelphia into vinegar before opening them—the 18th-century equivalent of hand sanitizer. In the case of yellow fever, of course, these precautions were unnecessary. In places with no mosquitoes, there was no real danger of contagion.

Some myths are common to both the current pandemic and the epidemic of 1793—like the idea that African Americans are immune or less susceptible to the disease. Acting on that belief, Rush employed black people as nurses and gravediggers for yellow-fever victims. In the end, African American and white Philadelphians died in similar proportions. A less pernicious but equally persistent misconception is that garlic has protective powers. Rush reported that people in Philadelphia “chewed garlic constantly.” During the current pandemic, one woman in China needed hospital treatment for a severe sore throat after consuming one and a half kilograms of raw garlic.

Eventually, what ended the yellow-fever epidemic was frost, which killed the mosquitoes. By November, the fever was disappearing, and those who had left or avoided Philadelphia—including President George Washington and Vice President John Adams—began cautiously to return.

One letter to the editor of a newspaper unwittingly hit upon what may have been the most valuable piece of advice. The author, identified as “A.B.,” noted that the recent rains had produced a lot of mosquitoes, which was “distressing to the sick, and troublesome to those who are well.” A.B. recommended pouring half a teaspoon of oil into the rainwater barrels where the insects bred, “which will quickly diffuse over the surface, and by excluding the air, will destroy the whole brood.” Whether anyone in Philadelphia paid attention is not recorded.

Meanwhile, Jean Deveze, a French doctor with experience treating yellow fever in the West Indies, was running a hospital for victims. He was convinced, rightly, that the disease was not contagious, and he rejected Rush’s techniques in favor of gentler remedies like “stimulants” and quinine—similar to modern treatments for yellow fever. But, partly because he was a foreigner and partly because victims were often beyond help by the time they arrived at the hospital, his efforts were largely overlooked. Today, in the midst of a global pandemic, perhaps some doctor or researcher who has discovered an effective weapon to fight COVID-19 is, like Deveze, toiling in obscurity.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.