The El Niño event coinciding with the 1878 epidemic was one of the strongest on record.

According to research published in the Bulletin of the American Meteorological Society in 1999, this provided the perfect conditions for A. aegypti to spread yellow fever.

El Niño is a band of warm water that develops off the Pacific coast of South America every 4 years or so. The phenomenon results in high rainfall, warm springs, and hot summers in southern U.S. states.

Between 1793 and 1905, there were nine devastating yellow fever epidemics. Seven coincided with a major El Niño episode.

But it would take until the end of the 20th century before scientists realized why outbreaks were so much worse in some years than others.

By 1911, improvements in rainwater storage and sanitation had denied the mosquito many of its former breeding grounds in open rain barrels and cisterns close to houses.

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“During its brief reign — July to October — its ravages were such as to completely paralyze both the social and commercial interests of a given city, and even an entire section of our country,” a U.S. Senate Report noted in 1911.

Yellow fever was a regular scourge of cities in the lower Mississippi River basin in the 18th and 19th centuries.

Around 100,000 people contracted the disease, and up to 20,000 people lost their lives. Some estimates put the economic cost as high as $200m .

In the summer of 1878, for example, the southern United States was struck by a catastrophic outbreak of yellow fever, a viral disease indirectly transmitted between people via the mosquito Aedes aegypti.

We know from past epidemics that changes in temperature, rainfall, and humidity can have profound effects on the spread of infectious disease .

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In the coming decades, ecological degradation, rising temperatures, and extreme weather events could intensify the threats to human health posed by viruses.

Predicting how future climate change will influence the spread of viral infections is fraught with difficulty. This is due to the complexity of interactions between climate, nature, and human activity.

But annual fluctuations in some viral infections, such as seasonal flu, and historical epidemics, such as yellow fever, provide some clues.

According to the Intergovernmental Panel on Climate Change, human activity has already caused approximately 1.0°C of global warming above pre-industrial levels. If warming continues at its current rate, temperatures will reach 1.5°C above these levels between 2030 and 2052.

As a result, there is likely to be more extreme weather, including more droughts, flooding, and heatwaves. Changes in temperature, rainfall, and humidity will have numerous knock-on effects on the world’s animals and ecosystems.

Among the species affected will be the animal hosts of viruses that also infect humans — or that have the potential to do so — and the insect “vectors” that transmit them.

There is no evidence that climate change played any role in the coronavirus pandemic, but there is intense debate about a possible role of different weather patterns.

Nonetheless, there are lessons to be learned about how future changes in human activity driven by climate change might increase the likelihood of viruses jumping from wild species into our own.

As happened with COVID-19, which is the infection caused by the novel coronavirus SARS-CoV-2, the leaps of these viruses between species can create new diseases to which humans have little immunity.

According to a report by the World Health Organization (WHO), “Climate change, one of the global environmental changes now underway, is anticipated to have a wide range of impacts upon the occurrence of infectious disease in human populations.”

It is possible to summarize the mechanisms through which it might influence the spread of viral disease as follows:

insect vectors

animal hosts

human behavior

the immune system

Insect vectors

Biting insects, such as mosquitoes, ticks, and sandflies, that transmit viral infections are cold-blooded. This means they are unable to regulate their body temperature, so external fluctuations strongly influence them.

A sudden, large increase in temperature might eliminate an insect vector, but it might benefit from smaller, incremental increases. Warmer conditions might improve breeding conditions, make food more abundant, increase activity, or extend its lifespan, for example.

In theory, increases in temperature due to climate change could potentially increase human exposure to insect vectors, or increase their biting rate.

There is a limited range of climatic conditions within which insects can survive and reproduce. A warming climate may, therefore, result in shifts in their geographical range or force them to evolve in some way to adapt.

These changes could result in “emerging infectious disease,” defined as an infection that has increased in incidence or spread to new regions or populations in the past 20 years.

A report published in 2008 in the journal Nature found that vector-borne infections accounted for around 30% of all emerging infectious diseases over the previous decade.

Worryingly, the increase to 30% represents a significant increase over previous decades.

The authors wrote: “This rise corresponds to climate anomalies occurring during the 1990s, adding support to hypotheses that climate change may drive the emergence of diseases that have vectors sensitive to changes in environmental conditions, such as rainfall, temperature, and severe weather events.”

Changes in precipitation

Experts predict climate change to increase rainfall in some regions and reduce it in others, with complex, unpredictable effects on vectors.

Increased precipitation could result in the development of more areas of still, open water. These areas, such as puddles and discarded containers, are perfect for the larval stages of vectors to grow in.

According to the WHO, wet, humid conditions may have caused past outbreaks of yellow fever and Dengue fever, both spread by the A. aegypti mosquito.

In some places, droughts could also increase opportunities for vectors to breed, as river beds dry up to leave stagnant pools, and as humans try to collect and store more rainwater in butts and reservoirs.

Experts think that a warm winter followed by a hot, dry summer in 1999 led to outbreaks of mosquito-borne West Nile virus in mid-Atlantic U.S. states through a complex web of ecological changes.

In addition to the increased availability of stagnant water for breeding, the ecological changes may have skewed the natural balance of nature in other ways. For example, there may have been fewer frogs and dragonflies around to eat the insect larvae.

Birds are the virus’s primary host, and their higher concentrations at the shrinking waterholes may have made them an easy target for biting insects.

Animal hosts

Infectious diseases that people catch from animals are known as zoonoses.

The authors of an article in the journal Annals of the American Thoracic Society point out that if climate change displaces wild animals, they will bring their zoonoses with them.

They write:

“Climate change may shift habitats and bring wildlife, crops, livestock, and humans into contact with pathogens to which they have had less exposure and immunity.”

Changes in rainfall and temperature, for example, can affect the availability of food eaten by animal hosts, such as bats, chimps, pangolin, and deer. The resulting changes in the size and range of their populations may bring them into closer contact with humans.

There is some evidence that this has happened in the past.

In late 1999 and early 2000, scientists in Los Santos in Panama identified the first ever cases in Central America of hantavirus pulmonary syndrome.

This potentially fatal lung disease is a zoonosis caused by a virus shed in the saliva, urine, and feces of rodents.

A report in Emerging Infectious Diseases pins the blame for the outbreak on a two- to three-fold increase in rainfall in Los Santos in September and October 1999, which led to an explosion in rodent numbers.

Excess rainfall may also indirectly promote the spread of enteroviruses that affect millions of people worldwide every year. Humans transmit enteroviruses, including poliovirus, coxsackie, and echovirus, to other people via the fecal-oral route.

For example, climate change can cause flash floods on land and sweep human sewage into the sea. When this happens, some of these viruses might contaminate shellfish, for example, leading to higher levels of disease in humans.

Human behavior

The U.S. Centers for Disease Control and Prevention (CDC) estimate that three out of every four new or emerging diseases come from animals.

Experts have linked the earliest cases of COVID-19 to the Huanan “wet” market in Wuhan province, China, where people sold wild animals for meat.

A new study published in Nature has confirmed that novel coronavirus was not made in a laboratory, as some conspiracy theories had suggested. Rather, its genome bears a striking resemblance to bat coronaviruses, and it is similar to coronaviruses that infect pangolins.

This is consistent with the theory that the virus spread to humans from bats via the pangolins sold in the Huanan market.

While there is no suggestion that climate change played any role in the emergence of COVID-19, it may have a knock-on effect on the type of human activity that brings wild animals and people into closer contact, particularly when food is in short supply.

For example, if crops fail and livestock perishes due to increased flooding, droughts, heatwaves, or pests, hunger may drive people to hunt and eat more wild animals.

Something similar may have led to the emergence of Ebola, a particularly infectious and deadly virus, in a village deep in the Minkebe Forest in northern Gabon in 1996.

Experts believe that the outbreak was due to the villagers killing an eating a chimpanzee. Scientists linked a later outbreak that began in 2007 in West Africa to eating fruit bats.

The destruction of pristine forest ecosystems by logging and other human incursions may also increase the risk that other viruses will leap from wild animals into people.

According to another study published in Nature, degraded habitats harbor more of the viruses that can infect humans. This may be because biodiversity loss “amplifies” viral infections in the remaining species.

The scientists write:

“In principle, loss of biodiversity could either increase or decrease disease transmission. However, mounting evidence indicates that biodiversity loss frequently increases disease transmission.”

Human immunity

In northern latitudes, influenza epidemics tend to occur between October and May, peaking in January and February.

In general, warm weather reduces the spread of flu, possibly because people are less likely to gather indoors in large groups.

Alternatively, warmer and more humid conditions may reduce the viability of respiratory viruses. So climate change may push seasonal outbreaks northward, where it’s cooler and drier.

There is no scientific consensus on whether warmer conditions in the coming decades will result in more or less severe flu epidemics.

Climate change may have more subtle effects, however.

An analysis of influenza in the U.S. between 1997 and 2013, for example, found that warm winters were followed by earlier, more severe flu seasons the next year.

The paper in PLOS Currents: Influenza suggests that mild winters may reduce “herd immunity” because fewer people are contracting the virus. This makes it easier for the virus to spread the following year, resulting in worse outbreaks.

The authors of a study published this year in IOPscience warn that rapid fluctuations in temperature — a characteristic of global warming — impair the immune system’s ability to fight off respiratory infections.

They found that rapidly changing weather in the fall has associations with more severe outbreaks of flu in the ensuing winter months.

The scientists write:

“[Climate models suggest] that the rapid weather variability in autumn will continue to strengthen in some regions of northern mid-latitudes in a warming climate, implying that the risk of an influenza epidemic may increase 20% to 50% in some highly populated regions in the later 21st century.”

The immune systems of young children and older adults seem to be particularly vulnerable to rapid temperature changes. Doctors write in Annals of the American Thoracic Society that spikes in childhood pneumonia in Australia are associated with abrupt falls in temperature.

Cause for optimism?

There is concern that a changing climate will bring more viral disease outbreaks. However, although outbreaks may become more frequent, science is in a better place to counter them.

Recent technological advances mean that scientists can develop and manufacture diagnostic tests and vaccines at a speed that would have been unthinkable just a decade ago.

However frustratingly slow the response to COVID-19 may feel at the moment, a situation such as this would have been worse a decade ago, when it could take 10-15 years to develop a vaccine. Now, scientists are hopeful of having a vaccine against SARS-CoV-2 within the next 12-18 months.

An analysis of infectious disease outbreaks published by Journal of the Royal Society Interface in 2014 concluded:

“Our data suggest that, despite an increase in overall outbreaks, global improvements in prevention, early detection, control, and treatment are becoming more effective at reducing the number of people infected.”

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