An Aedes aegypti mosquito is photographed through a microscope (Felipe Dana/AP)

In “A Christmas Carol,” Scrooge is visited by three ghosts, each with a warning. Like Scrooge’s ghosts, the recent outbreak of yellow fever in Africa reminds us of what was, shows us what is and foreshadows what will be. But whereas Scrooge heeds his warnings, ours don’t quite seem to be getting through. This inattention jeopardizes decades of progress in infectious-disease control for developed and developing countries alike.

The Ghost of Epidemics Past

Yellow fever is a virus spread by Aedes mosquitos (which also spread Zika, dengue and chikungunya). Although most cases are mild, about 15 percent of patients progress to a more serious stage marked by jaundice, from which yellow fever takes its name.

Until the 20th century, yellow fever was one of the world’s most feared diseases — a major killer and threat to commerce. Historical efforts to fight yellow fever laid the groundwork for our current public-health policies and disease-control strategies, both nationally and internationally.

Yellow fever inspired the passage of the first comprehensive public-health law in the United States, in the late 1700s, and the creation of the first local Board of Health and the Marine Hospital Service (which became the Public Health Service).

Internationally, repeated epidemics of yellow fever (as well as cholera) led to the International Sanitary Conferences in the late 1800s and early 1900s. These conferences were the first time that countries came together to coordinate infectious-disease-control policies. Ultimately, they produced the International Health Regulations, which are still in effect and were last updated in 2005.

The regulations specifically require that travelers be vaccinated for yellow fever before entering or leaving endemic countries, making it the only disease to be singled out in this way. It was also the first disease formally targeted for eradication, and it galvanized some of the earliest internationally coordinated mass-vaccination campaigns.

So, too, yellow fever inspired one of the original “global health partnerships” when the Rockefeller Foundation collaborated with Latin American governments on mosquito elimination; Rockefeller also developed the vaccine for yellow fever. Today, partnerships are a cornerstone of global-health governance, with the Gates Foundation playing a leading role. Similarly, the U.S. military’s fruitful involvement in infectious-disease research traces back to its efforts to combat yellow fever (and malaria) during the Spanish-American War and construction of the Panama Canal.

These efforts were highly successful, particularly in the Americas and West Africa. But ironically, while the strategies used to fight yellow fever lived on, the disease itself faded from policymakers’ minds. Mass-vaccination campaigns in West Africa ended in the 1960s. Mosquito control in the Americas became lax. In the words of World Health Organization Director-General Margaret Chan,  “for all practical purposes, yellow fever had been forgotten.”

The Ghost of Epidemics Present

Except yellow fever never went away. It remains endemic in sub-Saharan Africa and retains the potential to go wherever Aedes mosquitos do, meaning that it could reappear in the Americas or spread to Asia. As more people went without vaccinations, the number of cases began to climb; it now causes more than 150,000 severe cases and 60,000 deaths annually.

In 2006, the WHO and Gavi (the public-private vaccine alliance) moved to include yellow fever among routine childhood immunizations and resumed mass-vaccination campaigns. This initiative has reached 95 million people in Africa. But because of lack of political will and funding, another 705 million people remain vulnerable.

The current outbreak began in Angola and has spread to the Democratic Republic of Congo and Uganda. As of May 26, there are more than 2,600 suspected cases and 300 deaths. Additionally, eleven cases have been reported in China — a first in Asia. In each case, the outbreak was spread by non-immunized travelers, contravening International Health Regulations on vaccinations.

In May, the WHO convened a committee of experts to determine whether the yellow-fever outbreak constitutes a “public health emergency of international concern.” Although the committee members judged that it does not rise to that level yet, they “emphasized the serious national and international risks” and called for “intensified” action — especially enforcing the accination requirement.

As Chan suggested, the most “brutal” part of this outbreak is that we have an inexpensive, highly effective vaccine that confers lifetime immunity with a single dose. But we did not make adequate use of it in the past, and we do not have enough of it now.

Since a previous vaccine shortage in 2000, the WHO, along with its partners, has maintained an emergency stockpile. But the stockpile contains only 6 million doses, and it has already been exhausted. Vaccine manufacturers are working around the clock to replenish supplies, but they lack the surge capacity to increase production and accelerate distribution as needed in a crisis.

The Ghost of Epidemics Yet To Come

The Ebola and Zika outbreaks have prompted many discussions about how national governments and the international community should prepare for and respond to epidemics. The yellow-fever outbreak echoes some of the omens conveyed by these other epidemics but also contains three crucial warnings of its own.

Like Ebola, yellow fever illustrates how urbanization and new patterns of mobility are intensifying and accelerating the spread of disease. Like Zika, it shows how rising global temperatures will expand the range of “tropical” diseases, putting more places and people at risk. Today, most Americans view mosquito bites as annoying; in the future, we may learn to view them as dangerous.

But to some extent, Ebola and Zika took us by surprise. Although the viruses themselves are not new, they appeared in unexpected places, spread in unanticipated ways and, in Zika’s case, caused unfamiliar harms.

In contrast, yellow fever sprung no surprises. We just weren’t paying attention. Thus, the outbreak’s first warning is to not overlook the familiar.

Our thinking about epidemics is informed by our fear of the unknown and of being unprepared. We rightly worry about the next pandemic flu or antibiotic-resistant bacteria. But that doesn’t mean we can neglect the familiar diseases, assuming that we know how to deal with them and can put that knowledge into practice in time and at scale. Chan cautions that “you cannot trust the past when planning for the future.” In other words, yellow fever reminds us that diseases we have “put behind us” can be just as dangerous as the ones we have never encountered.

Furthermore, much of energy and funding behind epidemic responses is generally directed toward the quest for new treatments and vaccines. We’ve see this, for example, with Ebola, Zika and HIV. Likewise, policymakers often rush to create new programs and institutions and pass new regulations to fill perceived governance gaps. The global response to HIV generated an unprecedented number of new institutions, including UNAIDS and the Global Fund. The SARS outbreak motivated countries to finally revise the International Health Regulations. Ebola prompted the new Pandemic Emergency Financing Facility and the WHO’s health emergency program, which radically changes how that agency is structured and operates.

But with yellow fever, these policy responses are redundant. There is a vaccine. The necessary regulations and programs are in place. We just haven’t been using, enforcing and supporting them.

Thus, the outbreak’s second warning calls attention to a different set of global health governance challenges and foreshadows the stumbling blocks that may confront other epidemic responses in the future. The systemic weaknesses in vaccine production, distribution, and stockpiling are not unique to yellow fever. But so far, the Ebola and Zika outbreaks have not exposed these weaknesses because we don’t have vaccines.

Finally, the yellow-fever outbreak warns us that, short of full eradication, no public-health victory is irreversible. Amid all of the political focus on improving our response to health emergencies, it demonstrates that effective global-health governance is not only about responding to present threats. It also requires maintaining vigilance and initiative even against diseases that no longer seem threatening. Otherwise, the future is going to look a lot like the past.

Mara Pillinger is a PhD candidate in political science at George Washington University. Follow her on Twitter @mplng.