3 charts show how extreme climate helped spread 218 infectious diseases, from leprosy to COVID-19

A worker sprays insecticide for mosquitoes at a village in Bangkok, Thailand, on December 12, 2017. Athit Perawongmetha/Reuters

• Climate change is expected to weaken humans, supercharge pathogens, and push both into closer quarters.
• A new study shows extreme weather, ocean changes, and land disruption have already helped spread more than 200 pathogens.
• Three charts show how extreme weather and environmental changes help spread disease.

The first outbreak of Nipah virus, a deadly disease spread by bats, may have started with a wildfire. 

In 1997 and 1998, catastrophic fires erupted across Indonesia, punctuating a long drought and, many scientists suspect, driving fruit bats from their forest homes into Malaysia. There, the story goes, the increased bat population was attracted to fruiting trees on pig farms.

Soon pig farmers began developing fevers, headaches, sore throats, vomiting, and brain swelling. People in Singapore who ate Malaysian pork also developed symptoms. All in all, more than 300 people fell ill, and more than 100 died. To curb the sickness, more than 1 million pigs were culled, and pig farming was permanently banned in some regions.

Nipah virus is just one of 218 infectious diseases that have spread more widely among humans because of climate extremes — floods, drought, heat waves, hurricanes, ocean chemistry, sea-level rise, or other environmental conditions that are sensitive to climate change — according to a study published in the journal Nature Climate Change on August 8.

"I can tell you that story with bats, I can tell you that story with birds, I can tell you that story with rats, mice, deer. And I can tell it to you with viruses and with bacteria. And I can tell it to you via heat waves, floods, wildfires, even hurricanes, things that actually force those species to move," Camilo Mora, a data scientist at the University of Hawaiʻi Manoa who led the study, told Insider.

Grey-headed flying foxes hang in an animal trainer's house in the outskirts of Bangkok, on September 10, 2009. Sukree Sukplang/Reuters

By assessing historical records of infectious diseases dating back to the Roman Empire, Mora's team catalogued cases of climate extremes facilitating the spread of 58% of known human pathogens.

"I was not expecting that high of a number," Mora said.

It's likely an undercount, he added, since it only includes instances that were documented in published papers. While not all of those cases can be attributed to the current human-caused climate change, 80% of the papers are relatively recent, published in the last 20 years. They build upon a mountain of evidence that the extreme changes brought on by rising global temperatures help spread infectious diseases through three major pathways.

Pathway 1: Extreme weather and land disruption spread disease by pushing animals and people closer together

Residents walk through flood waters left in the wake of Hurricane Irma in a suburb of Orlando, Florida, on September 11, 2017. Gregg Newton/Reuters

In Siberia in 2016, an outbreak of anthrax was traced to a decades-old reindeer carcass unearthed by melting permafrost. That's an extreme case of climate change creating new contact between humans and infectious diseases, but the phenomenon is widespread.

Extreme weather events, which are becoming more frequent and severe with climate change, can displace animal and bird populations, driving them closer to humans. The Nature study found that vector-borne illnesses — the ones carried by animals and insects — were the most aggravated by climate extremes.

Changes in land use — like deforestation — can drive animal populations into places people live, or bring humans into animal territory. In the eastern US, studies suggest carving up forest territories for development led to increased overlap between humans and ticks, facilitating the spread of Lyme disease.

Patients rest on stretchers in the Cholera Treatment Center of Diquini in Port-au-Prince, Haiti, on May 28, 2016. Andres Martinez Casares/Reuters

Extreme weather can put humans in close quarters with each other, as well. Hurricanes and cyclones often lead to outbreaks of cholera, norovirus, and other deadly illnesses. Such outbreaks were well-documented in the aftermath of Hurricane Katrina. Flooding can expose people to water-borne diseases like vibriosis. A 1995 analysis even found that the spread of leprosy in Malawi was associated not with population density, but with rainfall.

Pathway 2: Extreme heat and rain can supercharge pathogens

An Anopheles stephensi mosquito — a known vector of malaria — obtains blood from a human host through its pointed proboscis. Jim Gathany/CDC/Handout via Reuters

Mosquitoes thrive in high temperatures and heavy rainfall, which creates stagnant water where they can lay their eggs. The diseases they spread, like malaria, West Nile virus, and chikungunya, thrive as climate change increases temperatures and heavy-rainfall events across many parts of the globe.

The pathogens themselves can grow stronger in extreme conditions, too. Warming oceans are creating fertile breeding waters for vibrio bacteria, which show signs of increased virulence in heat, allowing them to cause more severe illness.

Extreme heat waves, for example, can kill off many infectious viruses, bacteria, fungi, and the creatures that spread them. Whatever survives, however, is adapted to extreme heat — including the fever our bodies produce to kill off pathogens.

"The ones that survive are going to survive 42 degrees Celsius, meaning that when they come and infect us, one of the main mechanisms for us to fight off these diseases and these pathogens is not effective at all," Mora said.

Pathway 3: Extreme weather weakens infrastructure and makes humans prone to disease

A paramedic checks a man's blood pressure at a tent encampment during a heat wave in Shoreline, Washington, on July 26, 2022. David Ryder/Getty Images

Humans and their infrastructure are more vulnerable to the devastating impacts of disease when they're compromised by extreme weather. Wildfire smoke, for instance, can irritate the lining of the lungs, cause inflammation, inhibit the immune system, and leave people more vulnerable to respiratory illnesses like COVID-19.

Extreme weather events like heat waves can affect access to healthcare, by making it difficult or dangerous for people to leave their homes, or by destroying necessary infrastructure. Just this summer, heat melted roads and airport tarmacs, buckled railways, and caused power outages.

People affected by rapid weather variability or extreme events like hurricanes or fires might be stressed out, leading to heightened cortisol levels that weaken their immune systems. Malnutrition, expected to become more widespread as the changing climate affects the world's bread baskets, has a severe negative impact on the immune system. 

Adapting infrastructure, emergency plans, and healthcare to these new extremes can reduce the spread of diseases. But the Nature study concludes that the pathogens boosted by climate threats "are too numerous for comprehensive societal adaptations." Instead, the authors write, their findings highlight "the urgent need to work at the source of the problem: reducing [greenhouse gas] emissions."

"Keep in mind that this is not some weird alien that is causing climate change," Mora said, adding, "It's the contribution of small things that you and I do, multiplied by almost 8 billion people."