Infectious disease refers to illnesses caused by pathogens (disease-causing bacteria, viruses, fungi, and parasites) that can be transmitted to a person, directly or indirectly. When this disease is transmitted to a human from an animal, we call it a zoonotic disease. When it’s transmitted by a biting insect or arthropod (such as a tick, flea, or mosquito), we often call this a vector-borne disease. Lyme disease, malaria, zika, and dengue are examples of vector-borne diseases. Zoonotic and vector-borne infectious diseases overlap and are part of complex environmental ecosystems influenced by human-driven environmental destruction and climate change.

According to an article in the journal Nature, ecological damage contributed to the development of 335 new infectious diseases between 1940 and 2004. [1] Not all of these affect humans directly, though many have implications for the human food supply and economy.

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Infectious diseases affected by climate change

Warmer weather and longer growing seasons are altering the behaviour and life cycles of animals, insects, and arthropods. [2] This includes the expansion of animal habitats, which allows pathogens to spread into geographical regions where they were previously uncommon. In North America, for example, ticks that carry the Lyme pathogen are estimated to travel north at a rate of 46km per year, introducing Lyme disease into regions where it was once rare. [3] Rising temperatures are also predicted to extend the tick feeding season by an extra month, significantly increasing the likelihood of passing the Lyme bacteria from infected ticks to humans. [4] [5]

Similarly, Kaye et al. (2024) applied mathematical modeling to examine the global movement of Aedes aegypti mosquitoes and showed that both human-driven climate change and natural climate variability strongly influenced the spread of diseases carried by these mosquitoes, including Dengue fever, Zika virus, and Chikungunya. [6] This means that disease outbreaks could occur sooner or in different global regions than expected, making public-health planning difficult.
 
Infectious diseases impacted by land-use

Climate change is not the only environmental force driving the spread of infectious diseases. How we use our land is also key. Alterations to landscapes can have downstream effects on vector populations, reservoir hosts (the animals that transmit infections to vectors), protective hosts (the animals that limit the impact of a vector-borne infection), and human exposure to pathogens.

For example, as suburban areas grow to encircle and breach forested areas, remaining forest fragments increasingly favour high-risk reservoir species and become unsuitable for protective species. In North America, the loss predators such as foxes reduces natural control of white-footed mouse populations, which are a known reservoir for the Lyme bacteria. [7]
 

In Southeast Asia, research from 2019 showed that people who live or work on agricultural land were more than twice as likely to be infected with zoonotic or vector-borne illnesses, including malaria, bush typhus, and spotted fever. [8] In India, Kyansanur Forest Disease - a tick-borne viral infection with a fatality rate of up to 4% - is spreading due to deforestation and forest fragmentation. [9]

Forced migration, whether due to natural disaster or socio-political factors, is another example of how land-use change alters human-animal interactions and facilitates the spread of zoonotic infectious diseases. [10]

Researchers and organizations apply a number of evolving frameworks to better understand how human risk from zoonotic and vector-borne diseases emerges from complex interactions between our environment, socio-political systems, and economy. The socio-ecosystemic framework, Planetary Health, and One Health perspectives reflect more recent attempts to integrate these disciplines and provide a more holistic understanding of disease emergence and transmission. [11-16]