How do pandemics end? History suggests diseases fade but are almost never truly gone

Instead of looking forward, we should be consulting the past.

History can tell us how pandemics end.
Andrew Redington/Getty Images

When will the pandemic end? All these months in, with over 37 million COVID-19 cases and more than 1 million deaths globally, you may be wondering, with increasing exasperation, how long this will continue.


Since the beginning of the pandemic, epidemiologists and public health specialists have been using mathematical models to forecast the future in an effort to curb the coronvirus's spread. But infectious disease modeling is tricky. Epidemiologists warn that “[m]odels are not crystal balls," and even sophisticated versions, like those that combine forecasts or use machine learning, can't 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 looking forward for clues, you can look back to see what brought past outbreaks to a close – or didn't.

Where we are now in the course of the pandemic

In the early days of the pandemic, many people hoped the coronavirus would simply fade away. Some argued that it would disappear on its own with the summer heat. Others claimed that herd immunity would kick in once enough people had been infected. But none of that has happened.

A combination of public health efforts to contain and mitigate the pandemic – from rigorous testing and contact tracing to social distancing and wearing masks – have been proven to help. Given that the virus has spread almost everywhere in the world, though, such measures alone can't bring the pandemic to an end. All eyes are now turned to vaccine development, which is being pursued at unprecedented speed.

Yet experts tell us that even with a successful vaccine and effective treatment, COVID-19 may never go away. Even if the pandemic is curbed in one part of the world, it will likely continue in other places, causing infections elsewhere. And even if it is no longer an immediate pandemic-level threat, the coronavirus will likely become endemic – meaning slow, sustained transmission will persist. The coronavirus will continue to cause smaller outbreaks, much like seasonal flu.

The history of pandemics is full of such frustrating examples.

Once they emerge, diseases rarely leave

Whether bacterial, viral or parasitic, virtually every disease pathogen that has affected people over the last several thousand years is still with us, because it is nearly impossible to fully eradicate them.

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 fully eradicated.

So success stories like smallpox are exceptional. It is rather the rule that diseases come to stay.

Take, for example, pathogens like malaria. Transmitted via parasite, it's almost as old as humanity and still exacts a heavy disease burden today: There were about 228 million malaria cases and 405,000 deaths worldwide in 2018. Since 1955, global programs to eradicate malaria, assisted by the use of DDT and chloroquine, brought some success, but the disease is still endemic in many countries of the Global South.

Similarly, diseases such as tuberculosis, leprosy and measles have been with us for several millennia. And despite all efforts, immediate eradication is still not in sight.

Add to this mix relatively younger pathogens, such as HIV and Ebola virus, along with influenza and coronaviruses including SARS, MERS and SARS-CoV-2 that causes COVID-19, and the overall epidemiological picture becomes clear. Research on the global burden of disease finds that annual mortality caused by infectious diseases – most of which occurs in the developing world – is nearly one-third of all deaths globally.

Today, in an age of global air travel, climate change and ecological disturbances, we are constantly exposed to the threat of emerging infectious diseases while continuing 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.

Plague caused past pandemics – and still pops up

Even infections that now have effective vaccines and treatments continue to take lives. Perhaps no disease can help illustrate this point better than plague, the single most deadly infectious disease in human history. Its name continues to be synonymous with horror even today.

Plague is caused by the bacterium Yersinia pestis. There have been countless local outbreaks and at least three documented plague pandemics over the last 5,000 years, killing hundreds of millions of people. The most notorious of all pandemics was the Black Death of the mid-14th century.

Yet the Black Death was far from being an isolated outburst. Plague returned every decade or even more frequently, each time hitting already weakened societies and taking its toll during at least six centuries. Even before the sanitary revolution of the 19th century, each outbreak gradually died down over the course of months and sometimes years as a result of changes in temperature, humidity and the availability of hosts, vectors and a sufficient number of susceptible individuals.

Some societies recovered relatively quickly from their losses caused by the Black Death. Others never did. For example, medieval Egypt could not fully recover from the lingering effects of the pandemic, which particularly devastated its agricultural sector. The cumulative effects of declining populations became impossible to recoup. It led to the gradual decline of the Mamluk Sultanate and its conquest by the Ottomans within less than two centuries.

That very same state-wrecking plague bacterium remains with us even today, a reminder of the very long persistence and resilience of pathogens.

Hopefully COVID-19 will not persist for millennia. But until there's a successful vaccine, and likely even after, no one is safe. Politics here are crucial: When vaccination programs are weakened, infections can come roaring back. Just look at measles and polio, which resurge as soon as vaccination efforts falter.

Given such historical and contemporary precedents, humanity can only hope that the coronavirus that causes COVID-19 will prove to be a tractable and eradicable pathogen. But the history of pandemics teaches us to expect otherwise.The Conversation

Nükhet Varlik, Associate Professor of History, University of South Carolina

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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An international team of astronomers has conducted the biggest survey of stellar nurseries to date, charting more than 100,000 star-birthing regions across our corner of the universe.

Stellar nurseries: Outer space is filled with clouds of dust and gas called nebulae. In some of these nebulae, gravity will pull the dust and gas into clumps that eventually get so big, they collapse on themselves — and a star is born.

These star-birthing nebulae are known as stellar nurseries.

The challenge: Stars are a key part of the universe — they lead to the formation of planets and produce the elements needed to create life as we know it. A better understanding of stars, then, means a better understanding of the universe — but there's still a lot we don't know about star formation.

This is partly because it's hard to see what's going on in stellar nurseries — the clouds of dust obscure optical telescopes' view — and also because there are just so many of them that it's hard to know what the average nursery is like.

The survey: The astronomers conducted their survey of stellar nurseries using the massive ALMA telescope array in Chile. Because ALMA is a radio telescope, it captures the radio waves emanating from celestial objects, rather than the light.

"The new thing ... is that we can use ALMA to take pictures of many galaxies, and these pictures are as sharp and detailed as those taken by optical telescopes," Jiayi Sun, an Ohio State University (OSU) researcher, said in a press release.

"This just hasn't been possible before."

Over the course of the five-year survey, the group was able to chart more than 100,000 stellar nurseries across more than 90 nearby galaxies, expanding the amount of available data on the celestial objects tenfold, according to OSU researcher Adam Leroy.

New insights: The survey is already yielding new insights into stellar nurseries, including the fact that they appear to be more diverse than previously thought.

"For a long time, conventional wisdom among astronomers was that all stellar nurseries looked more or less the same," Sun said. "But with this survey we can see that this is really not the case."

"While there are some similarities, the nature and appearance of these nurseries change within and among galaxies," he continued, "just like cities or trees may vary in important ways as you go from place to place across the world."

Astronomers have also learned from the survey that stellar nurseries aren't particularly efficient at producing stars and tend to live for only 10 to 30 million years, which isn't very long on a universal scale.

Looking ahead: Data from the survey is now publicly available, so expect to see other researchers using it to make their own observations about stellar nurseries in the future.

"We have an incredible dataset here that will continue to be useful," Leroy said. "This is really a new view of galaxies and we expect to be learning from it for years to come."

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