The conversation
How do pandemics end? History suggests that diseases disappear, but almost never really disappear
When will the pandemic end? In all these months, with more than 37 million cases of COVID-19 and more than 1 million deaths globally, you may be wondering, with increasing exasperation, how long it will continue. Since the start of the pandemic, epidemiologists and public health experts have been using mathematical models to predict the future in an effort to stem the spread of the coronvirus. But modeling infectious diseases is complicated. Epidemiologists warn that “[m]odels are not crystal balls, ”and even sophisticated versions, such as those that combine predictions or use machine learning, cannot necessarily reveal when the pandemic will end or how many people will die. As a historian who studies disease and public health, I suggest that, instead of being anxious for clues, you can look back to see what brought the outbreaks of the past to an end – or not. Where we are now in the course of the pandemic In the early days of the pandemic, many people expected the coronavirus to simply disappear. Some argued that it would disappear on its own in the summer heat. Others claimed that collective immunity would increase when enough people were infected. But none of this happened. A combination of public health efforts to contain and mitigate the pandemic – from rigorous testing and contact tracking to social detachment and wearing masks – has proven to help. Given that the virus has spread to almost the entire world, however, such measures alone cannot end the pandemic. The hope now is vaccines, which were developed at an unprecedented rate. However, experts tell us that even with successful vaccines and effective treatment, COVID-19 may never go away. Even if the pandemic is contained in one part of the world, it is likely to continue elsewhere, causing infections elsewhere. And even if it is no longer an immediate pandemic threat, the coronavirus is likely to become endemic – meaning that slow and sustained transmission will persist. The coronavirus will continue to cause minor outbreaks, much like seasonal flu. The history of pandemics is full of frustrating examples. Once diseases arise, they rarely come out. Whether bacterial, viral or parasitic, virtually all pathogens that have affected people in the past thousands of years are still with us, because it is almost impossible to eradicate them entirely. The only disease that has been eradicated through vaccination is smallpox. Mass vaccination campaigns led by the World Health Organization in the 1960s and 1970s were successful and, in 1980, smallpox was declared the first – and still the only – human disease to be completely eradicated. Therefore, success stories like smallpox are exceptional. It is rather the rule that diseases come to stay. Consider, for example, pathogens like malaria. Transmitted via parasite, it is almost as old as humanity and still carries a heavy burden of disease today: there were about 228 million cases of malaria and 405,000 deaths worldwide in 2018. Since 1955, global programs to eradicate malaria, aided by the use of DDT and chloroquine, has brought some success, but the disease is still endemic in many countries of the Global South. Likewise, diseases like tuberculosis, leprosy and measles have been with us for several millennia. And despite all efforts, immediate eradication is not yet in sight. Add relatively younger pathogens such as HIV and the Ebola virus to this mix, along with influenza and coronavirus, including SARS, MERS and SARS-CoV-2, which causes COVID-19, and the overall epidemiological picture becomes clear. Research on the global disease burden has found that annual mortality from infectious diseases – most of which occurs in the developing world – is almost a third of all deaths in the world. Today, in an era of global air travel, climate change and ecological disturbances, we are constantly exposed to the threat of emerging infectious diseases, while we continue to suffer from much older diseases that remain alive and well. Once added to the repertoire of pathogens that affect human societies, most infectious diseases are here to stay. The plague has caused previous pandemics – and it still arises. Even infections that now have effective vaccines and treatments continue to claim lives. Perhaps no disease can help illustrate this point better than plague, the most deadly infectious disease in human history. His name remains synonymous with horror to this day. Plague is caused by the bacteria Yersinia pestis. Countless local outbreaks and at least three documented plague pandemics have occurred in the past 5,000 years, killing hundreds of millions of people. The most notorious of all pandemics was the Black Death of the mid-14th century. However, the Black Death was far from being an isolated explosion. The plague has returned every decade or even more often, each time striking already weakened societies and taking its toll for at least six centuries. Even before the health revolution of the 19th century, each outbreak gradually died out over months and sometimes years, as a result of changes in temperature, humidity and availability of hosts, vectors and a sufficient number of susceptible individuals. Some societies recovered relatively quickly from the losses caused by the Black Death. Others never did. For example, medieval Egypt has failed to fully recover from the persistent effects of the pandemic, which has particularly devastated its agricultural sector. The cumulative effects of declining populations have become impossible to recover. This led to the gradual decline of the Mamluk sultanate and its conquest by the Ottomans in less than two centuries.[Deep knowledge, daily. Sign up for The Conversation’s newsletter.]That same state-destroying plague bacteria remains with us today, a reminder of the pathogens’ long persistence and resilience. Hopefully, COVID-19 will not persist for millennia. But even with successful vaccines, no one is safe. The policy here is crucial: when vaccination programs are weakened, infections can come back in full force. Just look at measles and polio, which resurface as soon as vaccination efforts falter. Given these historical and contemporary precedents, humanity can only hope that the coronavirus that causes COVID-19 will become a treatable and eradicable pathogen. But the history of pandemics teaches us to expect the opposite. This article was republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts. Read more: * How 3 previous pandemics triggered massive social changes * Seat belts and smoking rates show that people eventually adopt healthy behaviors – but it may take a while that we don’t have during a pandemic * Compare the 1918 flu pandemic and COVID-19 carefully – the past is not a prediction Nükhet Varlik does not work, consult, own shares or receive funding from any company or organization that would benefit from this article and has not disclosed relevant affiliations beyond his academic appointment.