In 19th-Century Gibraltar, Survivors of a Deadly Virus Used ‘Fever Passes’ to Prove Their Immunity

Should historic health officials’ response to yellow fever outbreaks on the Iberian Peninsula serve as a model for modern pandemic management strategies?

Illustration of vaccine passports overlaid on map of Gibraltar
Survivors received “fever passes” that certified their immunity, allowing them increased freedom of movement at a time when a substantial portion of the population was being held under strict quarantine. Photo illustration by Meilan Solly / Photos via Wikimedia Commons, courtesy of Lawrence Sawchuk and Lianne Tripp

In August 1804, a shopkeeper named Santo entered the gates of Gibraltar, unaware that a pernicious virus was coursing through his blood. He had taken a trip to neighboring Spain, where, it seems, his skin was pricked by a mosquito carrying yellow fever. Within a day of his return, Santo had fallen ill—the first documented victim in Gibraltar of a disease that would wreak havoc on the Mediterranean fortress town during the early years of the 19th century.

Over the course of just four months in 1804, yellow fever claimed the lives of more than 2,200 people in Gibraltar, an estimated quarter of the permanent residents and military personnel who lived within the fortress. This epidemic was followed by four others, fueling repeated bouts of fear and despair. Time and again, residents watched as their loved ones and neighbors succumbed to an illness that, in its severest forms, causes an alarming litany of symptoms: jaundice—a yellowing of the skin and eyes that gives the virus its name; black vomit; bleeding from the eyes, nose and mouth. Health officials tried to stamp out the disease but didn’t understand how yellow fever was transmitted. It was only at the turn of the 20th century that the Aedes aegypti mosquito was revealed to be a vector of yellow fever, silently transmitting the virus as it flits from person to person, sucking up its meals.

Illustration showing the development of yellow fever
Illustration showing the development of yellow fever in a patient in Cadiz, Spain, in 1819 Wellcome Collection via Wikimedia Commons under CC BY 4.0

But authorities were quick to recognize one important truth: People who contract yellow fever and survive are not vulnerable to subsequent infections. Today, this concept is known as immunity; in the 19th century, the term “non-liability” was used. By Gibraltar’s fifth epidemic in 1828, an innovative measure had been put in place to accommodate those with protection against yellow fever. Survivors were granted “fever passes” that certified their non-liability, allowing them increased freedom of movement at a time when a substantial portion of the population was being held under strict quarantine.

This concept resonates today, as countries wade through the Covid-19 pandemic and grapple with the challenges of easing lockdown restrictions while the virus continues to mutate, infect and spread. As part of their reopening plans, some governments and businesses have mandated “vaccine passports”—documents, either digital or paper, that prove vaccination status—to ensure that only those with a high degree of protection against Covid-19 are able to cross borders and access certain public spaces, like restaurants, movie theaters and concert venues.

Documents testifying to an individual’s good health have long been deployed during times of rampant sickness. As far back as the 15th century, travelers could carry “health passes” certifying that they came from a location free of the plague. According to a recent paper published in the journal BMJ Global Health, however, the earliest evidence of passports showing that the holder is immune to a disease comes from Gibraltar 200 years ago. 

“Having this passport gave you the freedom ... to be able to do something that was almost normal, and that is to move about somewhat freely,” says study co-author Larry Sawchuk, an anthropologist at the University of Toronto Scarborough whose research focuses on the population health of Gibraltar and the Maltese Islands.

Fever pass for Anna
1828 yellow fever pass for 14-year-old Anna Courtesy of Lawrence Sawchuk and Lianne Tripp

Located at the southern tip of the Iberian Peninsula, Gibraltar is a small strip of land dominated by a soaring promontory—the famed Rock of Gibraltar. For hundreds of years, this slip of a territory was coveted by diverse nations for its strategic location next to the Strait of Gibraltar, the only route into the Mediterranean via the Atlantic Ocean. Gibraltar was occupied by the Moors in the eighth century C.E.; captured by Spanish forces in 1462; and taken by the British in 1704, during the War of the Spanish Succession.

When yellow fever first struck in the early 1800s, “the Rock,” as Gibraltar is known colloquially, was a closely guarded garrison town under the absolute authority of a British military governor. Residents lived within the walls of an imposing fortress that had been built, modified, damaged and repaired over centuries of tumultuous history. Police surveilled the population, and the gates of the town were constantly guarded by soldiers. Permits were required to leave and enter these gates, which opened at daybreak and closed at dusk. 

“Under that sort of system, the citizen had absolutely no rights,” says study co-author Lianne Tripp, an anthropologist at the University of Northern British Columbia who studies health and disease in the Mediterranean in the 19th and 20th centuries. “They had to do whatever was needed to be done to serve the fortress.”

In spite of the restrictive nature of life on the Rock, Gibraltar was an important trade hub and a pulsing, crowded, cosmopolitan town. People from Italy, Spain, Morocco, England and other diverse locations flocked to Gibraltar, drawn in by its free port and the promise of year-round employment that couldn’t be found in the nearby south of Spain, where jobs tended to be seasonal.

1803 map of Gibraltar
1803 map of Gibraltar Public domain via Wikimedia Commons

The virus that would come to plague the fortress likely originated in the rainforests of Africa, making its way to the Western Hemisphere via ships carrying enslaved people in the 17th century. Yellow fever eventually spread to Europe, possibly hitching a ride on trade ships coming from the Americas. A 1730 epidemic in Cadiz, Spain, killed 2,200 people and was followed by outbreaks in French and British ports. Yellow fever may have been introduced to Gibraltar in 1804 by someone coming from Spain—Santo, perhaps, or another traveler who escaped the notice of medical authorities. When it breached the walls of the fortress, the virus found a perfect storm of conditions that allowed it to proliferate to devastating effect.

The colony was, for one, notoriously overcrowded. Its residents, many of them impoverished, packed into the fortress, living in “patios,” or multi-tenant buildings that shared an open common area. “You’d have a room with ten people in it, and they would sleep in that room, and they were separated by about two inches,” says Sawchuk. For Aedes aegypti mosquitoes, which do not fly particularly long distances, these dense urban conditions served up an easy smorgasbord of human hosts. Late summer heat and humidity also provided ideal temperatures for the insects to thrive, and an ample supply of standing water offered plenty of breeding grounds; no springs or rivers run through Gibraltar, so residents relied on rainfall for drinking water, which they collected in buckets and jugs.

Most people in Gibraltar had no previous exposure to yellow fever and thus no immunity against it. The virus usually causes mild flu-like symptoms, but some patients who seem to recover enter a toxic second phase that kills up to 50 percent of patients. In Gibraltar, the dead piled up so quickly that coffins could be produced fast enough for only one out of every four bodies. Corpses were heaped onto carts that trundled through the town, a haunting reminder to the living that they were surrounded by death. But the carts couldn’t keep up. One journal from the period records a young woman “throwing her dead father out of the chamber window,” perhaps knowing that his body would likely not be collected anytime soon.

Fever pass for Juan
1828 yellow fever pass for 17-year-old Juan Courtesy of Lawrence Sawchuk and Lianne Tripp

The epidemic slowed its fatal march through Gibraltar once cold weather set in and yellow fever’s bloodsucking vectors died off. Local authorities who had been blindsided by the virus established a Board of Public Health and were ready to act when a smaller series of epidemics broke out in 1810, 1813 and 1814. 

One significant measure involved the creation of a quarantine encampment on the isthmus between Gibraltar and Spain, an area known as the Neutral Ground. The site was established in 1810, quickly and secretly. In the dead of night, authorities rapped on the doors of households affected by yellow fever and forcibly escorted the sick to the Neutral Ground. They stayed there, sequestered in tents and monitored by guards, until the epidemic had waned.

Later, in 1814, a cohort of civilian volunteers was enlisted to keep track of the population’s health. Every day, the volunteers went door-to-door within the fortress, making note of residents who were sick and those who remained vulnerable to the virus. These observers recorded overcrowding and uncleanliness and doused homes that were affected by yellow fever with lime and hot water. 

Some of these protocols were quite innovative. Tripp notes, for example, that the practice of conducting door-to-door surveys during public health crises is typically associated with John Snow, a physician who mapped out cholera cases in London in the mid-1850s, nearly three decades after Gibraltar’s last yellow fever epidemic. Still, authorities on the Rock were basing their management strategies on two incorrect theories of yellow fever transmission: They believed the disease spread directly from person to person or that it dispersed through foul air emanating from rotting filth. It is largely coincidental that, after the first epidemic in 1804, Gibraltar managed to avoid a second severe epidemic for nearly 25 years. Factors like ample rainfall, which was used to cool feverish bodies, may have done more to temper yellow fever deaths than quarantines or sanitization efforts, according to Sawchuk.

Map of the Neutral Ground
Map of the Neutral Ground, where yellow fever patients—and later those susceptible to yellow fever—were quarantined Courtesy of Lawrence Sawchuk and Lianne Tripp

Despite officials’ best efforts, yellow fever returned to the fortress in fall 1828 with a virulence that recalled the first epidemic, ultimately killing more than 1,600 people. As the crisis raged, health officials decided to tweak one of their key management protocols. Instead of quarantining the sick in the Neutral Ground, they ordered all those who had not been infected by the virus to immediately relocate to the encampment, along with the rest of their households. 

Scholars cannot definitively say why this change in policy was made, but it required a “formidable” level of contact tracing, write Sawchuk and Tripp in their paper. Authorities relied on meticulous house-to-house surveys to identify and segregate people lacking immunity from those who had survived past epidemics. The measure was likely life-saving for reasons that officials wouldn’t have understood. Unlike the densely concentrated town, the Neutral Ground wasn’t filled with barrels of standing water where mosquitoes could breed. Windy weather on the isthmus also kept the insects away.

Not all of the 4,000 people relocated to the encampment needed this protection. Some had survived previous epidemics but were carted off to the Neutral Ground because they lived in the same household as an individual who had never been sick. The Neutral Ground wasn’t a particularly pleasant place to be: “You’re living in a tent or a shed,” Sawchuk says. “There’s no escaping everybody looking at you, hearing exactly what you’re saying. For four months ... that would drive me a little crazy.” Life in the encampment would have been terribly dull, he adds. Those quarantined at the site were kept from their jobs, their friends, the bustle of the town—until authorities began issuing passes that allowed yellow fever survivors to travel in and out of the encampment and even reside in the town.

Only two such fever passes are known to survive today. Housed in the Gibraltar National Museum, they are printed on small squares of yellowing paper, with blank spaces for a physician to fill out the patient’s name, age and religious affiliation. The documents belonged to a pair of teenagers, Juan and Anna; their last name is difficult to decipher, but they were likely siblings. Juan was 17 and Anna was 14 at the time of Gibraltar’s last yellow fever outbreak. A physician’s signature certified that each had “passed the present Epidemic Fever.”

1878 illustration of Spanish soldiers returning from Cuba being fumigated to protect against yellow fever
1878 illustration of soldiers returning from Cuba being fumigated to protect against yellow fever Public domain via Wikimedia Commons

Experts don’t know how many fever passes were issued in 1828, but the fact that the documents were standardized and printed suggests there were “a good number of them,” says Tripp. The relief that came with obtaining one of these passes, particularly considering that residents were not afforded the luxury of quarantining in their own homes, must have been palpable. “[Fever passes] gave you the freedom to escape the monotony of living in this encampment,” Sawchuk says.

Modern vaccine passports are a comparable measure intended to ease restrictions for those with protection against Covid-19. But the case study of Gibraltar does not provide easy answers to the thorny questions raised by the vaccine passport system. After all, 19th-century Gibraltar was clearly not a free state. Even prior to its spate of epidemics, citizens’ movement was controlled through permits required to enter and leave the fortress. Fever passes may very well have seemed like business as usual to residents of the garrison town.

Today, by contrast, vaccine passports have caused considerable hand-wringing among ethicists, policy makers and citizens. Proponents argue that the documents allow individuals to safely return to gathering indoors, which comes with numerous benefits, like reuniting families and reviving the global economy. But good-faith critics have voiced concerns that the passports violate civil liberties and open the door for “chilling” invasions of privacy and surveillance.

Many of the fundamental mitigation strategies that we put in place have been around for hundreds of years.

Another fear is that vaccine passports worsen existing inequalities both within countries and on a global level. Requiring such documents for international travel “restrict[s] the freedom of people in low- and middle-income countries most because they have the least vaccine access,” says Nancy S. Jecker, an expert on bioethics and humanities at the University of Washington School of Medicine who authored a recent paper on vaccine passports and health disparities. She adds that domestic vaccine passports are also problematic because they have “unfair and disproportionate effects” on segments of the population that do not always have equal access to Covid vaccines, like low-income groups and racial and ethnic minorities.

Jecker does not broadly oppose the idea of a health pass; for domestic travel, she supports a “flexible” system that allows people to show proof of vaccination, past Covid infection or a recent negative test. “There’s a lot of emphasis in my field [on] this notion of respect for individual autonomy,” she says. “And it’s really not the value we need right now as a standalone. We need to balance it against other values like public health.”

Officials in 19th-century Gibraltar wouldn’t have been particularly concerned about striking this balance, and both Sawchuk and Tripp acknowledge that the colony is an imperfect model for contemporary pandemic management strategies. “It was a different time,” Sawchuk says, “a different disease.” But the researchers believe it is important to reflect on Gibraltar’s historic epidemics, which show that key experiences during times of public health crises are repeated across the centuries. 

“Many of the fundamental mitigation strategies that we put in place have been around for hundreds of years,” says Tripp, citing the examples of quarantines and health passports. “The idea of immunity has been around even before we understood how diseases were transmitted. So when we talk about unprecedented times, [today] really isn’t that unprecedented.”

Get the latest History stories in your inbox?

Click to visit our Privacy Statement.