Have we learned the wrong lessons from the Ebola outbreak?

Last Thursday, the World Health Organization declared the end to two horrific years of the West African Ebola epidemic.

Later on the same day, the Ministry of Health in Sierra Leone announced that a patient with Ebola died in the Tonkilli region of that country.

Perhaps the most disconcerting aspect of the new case in Sierra Leone was not that it occurred so soon after WHO's proclamation, but that Ebola wasn't diagnosed until after the patient died.

The patient was a young woman who developed symptoms at the beginning of the year after traveling to an area in that country that was one of the last hotspots to be declared disease-free. When she came to a local hospital for care, she had classic symptoms of Ebola, such as vomiting and diarrhea. Yet she was not diagnosed with the disease.

What's more, the health care worker who drew her blood did not wear the appropriate personal protective equipment and the woman's blood sample may not even have been tested for Ebola. She was eventually discharged to die at home rather than being isolated, and dozens of other people have been exposed. Currently, some 100 people who may have had contact with her are under quarantine.

The juxtaposition of the upbeat announcement and the sad news is a reminder of how difficult it is to bring an epidemic to an end — and what we need to keep in mind about this particular epidemic and any future outbreaks of other diseases.

How and why do human epidemics end? There are several reasons. Cases drop when there are no more susceptible humans left to infect, when we get better at controlling the spread, when the organism that causes the disease mutates to be less lethal, or when one of the conditions that allowed the disease to survive in nature changed — perhaps an animal that carries the disease moved or died off or there was a change in weather conditions that had been favorable to the spread.

In most cases, a combination of these factors plays a role. So predicting the end of an epidemic can be a difficult endeavor because it requires perfect knowledge of all of the above factors. That's simply not possible. Hence, the best public health officials can say is that there is no more evident human-to-human transmission noted. And to be fair, this has been WHO's message after each country ends its surveillance period.

Some scientists believe the end of the wide-scale epidemic marks the beginning of recurrent small groups of cases in this region for months and potentially years. The virus can take months to clear completely from the body of a survivor. There are nearly 17,000 Ebola survivors in West Africa. They can transmit the infection to those around them — although the frequency of such transmission is low. The transmission may be possible with sexual contact. It can also occur if the patient's immune system is depressed for other reasons, and dormant Ebola virus is able to start replicating more actively.

To halt future cases, health workers must provide care, counseling and surveillance of symptoms that may represent Ebola reactivation among survivors. How long should such surveillance continue — in other words, how long will it take for the immune system to clear out the last remnants of the virus? The answer is we don't know. The leading theory suggests this should be in the order of months rather than years. There are, however, other reasons for continued concern about Ebola in West Africa. The conditions that allowed this virus to jump to the human population and spread so quickly have not all changed. We have not run out of people who are susceptible to being infected, although the pool is smaller after accounting for those who developed immunity to the virus naturally after contracting the disease. We have also made strides to shrink this pool even further with the success of one of the Ebola vaccine candidates, Merck's rVSV-ZEBOV. Given immediately after exposure, it was shown to all but remove the chances of developing Ebola.

We have still not definitively proved where this virus lives in nature when it is not infecting humans. Fruit bats are the leading contenders, but more research and surveillance of the natural world are needed. More research is also needed to understand what exact conditions allow the virus to jump from animals to humans. Hence we are a long way from being able to prevent these phenomena from happening.

And the case in Sierra Leone last week brings up another point: The systemic weaknesses in public health infrastructure that allowed the virus to spread like wildfire still exist. If we were able to stop this epidemic, it was not because we rebuilt the public health systems in West Africa but because massive resources were poured into immediate response and heightened surveillance, with an immense amount of education and policies that echoed wartime precautions, like roadblocks that restricted the movement of entire countries.

Having worked for months in Sierra Leone over the past two years, I can bear witness to the toll this epidemic has taken on affected communities. Nothing could be more tempting than to put this behind us.

Despite WHO's caution that there could be flare-ups, the declaration of the end to the epidemic may signal that the behavioral changes and the heightened surveillance in West Africa are no longer necessary. This is particularly true when the entire enterprise was built on sheer will and very little permanent scaffolding to ensure that positive changes to the health care system stick.

So it is important to have continued vigilance in West Africa. But that is not the only lesson to come out of this Ebola outbreak. By focusing on the end of the epidemic, we miss the larger point. We cannot live as a world that moves from responding to one epidemic after another. Rather than thinking of beginnings and ends, we need continuous surveillance for threatening infectious diseases that are both known and yet to be discovered. We need to move from a culture of outbreak response to one that focuses on prevention. Small clusters of infectious disease cases are inevitable, but outbreaks and epidemics are preventable.

A stitch in time, though, is a hard sell. This past week also marked the release of a National Academy of Medicine sponsored commission's report on the Global Health Risk Framework, which addressed why epidemics are an immense and inevitable threat to our global security. The report outlined the responsibility of individual nations and the global community to invest in systems that can rapidly pick up new threats and be nimble enough to respond to them quickly. Per the report, it would take a $4.5 billion annual commitment to make these changes a reality. Although a daunting number, this investment is nowhere near close to the expected $60 billion in annual losses from pandemics. At the launch event for the report, the economist and president emeritus of Harvard University Larry Summers noted that the 1918 flu pandemic affected 7,000 times more people than the recent Ebola outbreak, striking a third of the world's population and killing 50 million to 100 million people — 3 to 5 percent of everyone on earth. We have made significant scientific advancements since 1918, but we also now live in a world where contagions move at the speed of modern travel and trade. And there are billions more of us, living in large crowded cities. In a study by Dr. Larry Brilliant, the epidemiologist and director of Skoll Global Threats Fund, 90 percent of epidemiologists polled said they expect a large pandemic in their children or grandchildren's lifetime, one that could affect over a billion people and cause global recession. In many ways, how we handled the Ebola crisis was a litmus test for our response to a large flu pandemic. And keep in mind that Ebola is much harder to spread over large geographical areas than respiratory viruses are. WHO has a difficult balance to strike between celebrating the hard work of stopping Ebola and continuing to urge a need for vigilance. As Albert Camus said in the novel The Plague, "There have been as many plagues as wars in history; yet always plagues and wars take people equally by surprise." The best way to keep new cases of infectious diseases from causing epidemics is to expect them and to invest in public health and research during "peace time" as much as we do during an outbreak. Nahid Bhadelia is an infectious disease physician at Boston Medical Center and the director of Infection Control at National Emerging Infectious Diseases Laboratory. Copyright 2019 NPR. To see more, visit https://www.npr.org.