by Willem van Schaik
Infectious diseases have been the most important cause of death in human history. Only since the late 19th century has our understanding of the microbial causes of infectious diseases led to the broad implementation of public health interventions to safely dispose of sewage and provide clean drinking water. From the 1950s onwards, the development of antimicrobial drugs and vaccines have been powerful additions to our arsenal against infectious diseases.
Collectively, these interventions and therapies have led to a significant decrease in deaths caused by infections globally, particularly in high-income countries. However, in the 21st century infectious diseases remain an important cause of death in the poorest countries in which the majority of the world’s population lives. Since late 2019, the dramatic events of the COVID-19 pandemic have highlighted that infectious diseases still have the capacity to destructively affect even the world’s most advanced economies and cause widespread suffering and death.
In this article, I will consider how climate change is already contributing to the global spread of infectious diseases. In addition, I will discuss whether new pathogenic microbes with pandemic potential are more likely to emerge and spread on a warming planet. Finally, I will outline potential interventions to rapidly detect and mitigate the spread of new and existing microbial threats.
An introduction to infectious diseases
Infectious diseases are caused by microbes, which include viruses, bacteria, fungi and parasites. The most important shared characteristic of all infectious diseases is that these microbes can spread between humans, thus causing the spread of the illness throughout populations. Sometimes this spread between two individuals can be direct and is described by the term ‘human-to-human’ transmission. Several important viral diseases, like measles and COVID-19, and the bacterial infection tuberculosis are important examples of illnesses that spread directly from human to human through the inhalation of droplets and/or aerosols that carry the infectious agents.
Other infectious diseases need an animal intermediary (termed a vector) for transmission. Important vector-borne infectious diseases are malaria, which needs the Anopheles mosquito to transmit, and plague, which is caused by a bacterium that can be transmitted via fleas and their bites. The vast majority of infectious diseases are zoonotic, meaning that the infectious microbe has, at some point in its natural history, jumped from an animal host to humans. SARS-CoV-2, the virus that causes COVID-19, appears to be an example of a zoonotic virus as current evidence points towards it having been carried by bats before it made the host jump to humans in late 2019. Other pandemic viruses are also zoonotic. HIV, the virus that causes the disease AIDS, is thought to have crossed from chimpanzees to humans in the 1920s. For influenza viruses, livestock and poultry are important hosts where new forms of the virus can emerge and subsequently spread to humans. In the vast majority of cases, the host jump of a virus to humans will not result in an infection that is characterised by sustained human-to-human transmission. However, the pandemics of the last century, including HIV, Zika, Ebola and COVID-19, have shown that novel viruses can still spread rapidly across human populations. The COVID-19 pandemic should thus not be seen as a unique ‘Black Swan event’, but rather as a warning to future generations that pandemics will always remain a threat to humanity. have shown that novel viruses can still spread rapidly across human populations. The COVID-19 pandemic should thus not be seen as a unique ‘Black Swan event’, but rather as a warning to future generations that pandemics will always remain a threat to humanity. have shown that novel viruses can still spread rapidly across human populations. The COVID-19 pandemic should thus not be seen as a unique ‘Black Swan event’, but rather as a warning to future generations that pandemics will always remain a threat to humanity.
Climate change changes the geographic range of vectors of infectious diseases
Global warming is changing the geographic range of mosquitoes and other insects that act as vectors for infectious diseases. This is now broadly recognised as a major threat to global public health. As an example, mosquitoes of the genus Aedes, particularly Aedes aegypti (the yellow fewer mosquito) and Aedes albopictus (the Asian tiger mosquito) are important vectors for viral infectious diseases including yellow fever, West Nile fever, Zika and Chikungunya. Since the 1960s, global trade in products like tyres and potted plants, which provide habitats for the development of larvae of A. aegypti and A. albopictus, has led to the introduction of these mosquitoes into North America and Europe. Winter temperature is likely to be the most important factor in determining how far these mosquitoes can spread to higher latitudes. With milder winters becoming more common on a warming planet, it is thus likely that these mosquitoes will spread further northwards in Europe and North America in the 21st century. In a worst-case ‘business-as-usual’ scenario for climate change, nearly a billion individuals will be newly exposed to these Aedes mosquitoes and the viruses that they can transmit. Current scenarios do not foresee a spread to the United Kingdom, but the mosquitoes are likely to become endemic across popular southern European destinations for tourism, thus potentially leading to increasing numbers of travel-related infections across Europe.
Ecosystem upheaval will increase human-animal interactions, facilitating virus-host jumps
An estimated 10,000 viruses that are currently circulating in wildlife are thought to be able to infect and spread among humans. These viruses are entirely unknown to science and humanity will probably only first know of them after they cross the species barrier to start causing disease in humans, similar to SARS-CoV-2 and HIV. On a warming planet, many of the ecosystems on our planet will undergo dramatic changes, which will lead to the movement of many animal species into new habitats. For example, the climatic changes in the 21st century are likely to contribute to increased droughts and larger, uncontrolled forest fires. These fires will not only directly affect the carbon balance on our planet, as tropical rainforests are among the most important carbon sinks on the planet, but will also lead to migration of animals from regions that are affected by fire. When animal populations move from their original habitats to new sites, there is an increased risk that they come into contact with humans, thereby increasing the risk of animal-to-human transmission of the viruses they carry. This is particularly relevant for bats, who are broadly recognised as an important reservoir for novel viruses. The capacity of bats to fly allows them to travel hundreds of kilometres within their lifetime, thus potentially contributing to the dissemination of novel viruses on continental scales after their original habitats have been disrupted by climate change.
Climate change and antibiotic resistant bacteria
The previous sections have mostly focused on viral diseases. However, infections caused by bacteria remain an important cause of disease and death globally. Since the mid-20th century, bacterial infections have been treated with antibiotics. Worryingly, bacteria that have evolved resistance to these drugs have become increasingly widespread. Infections caused by these antibiotic-resistant bacteria are becoming increasingly difficult to treat due to a lack of development of novel antibiotics. Antibiotic resistance can be regarded as a ‘slow pandemic’ that takes decades to spread, but can result in 10 million deaths per year by the year 2050. It may make medical interventions that are currently considered routine very risky as we would no longer be able to rely on antibiotics to suppress and cure infections.
Whereas antibiotic usage in human and veterinary medicine is broadly recognised as the leading driver for the emergence and spread of antibiotic resistance, recent research has shown that increasing temperatures in Europe and North America are associated with higher levels of drug-resistant bacteria. The mechanisms that are driving increased levels of antibiotic-resistant bacteria at higher ambient cultures are currently unknown. It is possible that the process of horizontal gene transfer, the exchange of DNA containing antibiotic resistance determinants between bacteria, is increased at higher temperatures. Global warming may thus directly contribute to the rapid spread of antibiotic-resistant bacteria across the planet.
A global response to infectious diseases in the age of climate change
To mitigate the impact of climate change on the spread of infectious diseases, we can build on the experience of the COVID-19 pandemic, which has made it abundantly clear humanity always needs to be prepared to go to battle with its microbial enemies. Rapid deployment of non-pharmaceutical interventions and the development of novel drugs and vaccines will remain cornerstones for an effective response to infectious diseases. In addition, global networks for surveillance and monitoring of infectious diseases, particularly in settings where humans and animals interact, e.g. in the vicinity of bat colonies, will need to be expanded to rapidly flag and identify new outbreaks of infections. Similar surveillance efforts are needed to map the spread of insect vectors of infectious diseases. These should be combined with public information campaigns to promote interventions to minimise insect bites and raise awareness when new infectious diseases are introduced through insects.
Climate change is predicted to first affect the poorest nations on the planet. These countries generally already have high rates of infectious diseases and often harbour ecosystems that can be conducive to animal-to-human transmission events of pathogens. On a warming planet, a truly global effort is thus urgently needed to stem the emergence and spread of new and existing infectious diseases.
 For further reading on this topic see: https://journals.plos.org/plosntds/article?id=10.1371/journal.pntd.0007213; https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3614918/ and
 For further reading on this topic see: https://www.bmj.com/content/371/bmj.m3389;
 For further reading on this topic see: https://amr-review.org/sites/default/files/160525_Final%20paper_with%20cover.pdf;
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6201249/ and https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2020.25.45.1900414