Changes in climate influence many infectious diseases.

Infectious diseases are caused by organisms that make us sick, also known as pathogens. Many viruses, bacteria, fungi, and parasites are pathogens. Each type of pathogenic organism has climatic conditions it prefers. Changes in weather patterns that favor the conditions needed by a pathogen―particularly in terms of average temperature, precipitation, and relative humidity―will positively affect its ability to survive and spread. Conditions that favor the animals (such as rodents, mosquitos, and ticks) that support pathogen reproduction and transmission will also help some infectious diseases spread.

Furthermore, as the climate warms, extreme weather events like heat waves, heavy rainfall, and droughts are becoming more common and more intense. These extreme events can create conditions that are favorable for disease spread. Extreme weather events that displace people, including floods and hurricanes, can also lead to circumstances that facilitate the spread of disease among people, such as through crowded conditions in emergency shelters and the disruption of healthcare services, including delays in routine childhood immunizations.

These are some of the ways climate change can increase the risk of epidemic infectious diseases―that is, of infectious diseases occurring in places where people were not previously at risk or where they face a higher risk of infection than before. However, some epidemic infectious diseases may not increase with global warming, and instead may decrease in frequency and severity. For example, some locations may become too hot or dry for disease-carrying mosquitoes to successfully survive and spread pathogens.

Climate change is one of many factors that can increase the spread of infectious diseases.

In addition to global warming, the lifecycles of pathogens and the animals that carry and transmit them are influenced by changes in land use and land cover. For example, in the parts of the world with the parasitic worms that cause schistosomiasis, dam construction can create the still, freshwater habitats preferred by snails that harbor the parasite, leading to outbreaks of the disease. Deforestation is linked to increases in infectious disease outbreaks around the world, particularly in tropical countries.

The spread of infectious disease agents is also affected by travel patterns. Air travel in particular makes it possible for an infectious disease agent to spread rapidly from area to area within a continent or from one continent to another. The speed at which this can happen was on display when the SARS-CoV-2 coronavirus emerged. The outbreak escalated from the first infections to a global pandemic in a few months.

The socioeconomic status and distribution of human populations are additional factors in the spread of infectious diseases. Population density is linked with increasing ease of transmission of airborne infections, waterborne diseases, and sexually transmitted infections. Poverty also affects the transmission of infectious diseases. A lack of basic necessities, such as shelter and good nutrition, can make people more vulnerable to infection.

These factors are closely connected with the effectiveness of the health system, access to health care, and availability of medical treatments. Sewage treatment and water disinfection are critical for providing safe drinking water and preventing waterborne disease outbreaks from cholera and typhoid. Vaccinations, antibiotic medicines, and regular medical care can also help stem the spread of infectious diseases, but their accessibility and affordability depend heavily on socioeconomic status and distance to health services.

Additional studies are needed to project how global warming could affect infectious disease outbreaks.

Cleary, many factors contribute to the spread of infectious diseases in addition to climate change. At the same time, climate change has many complex effects on the physical environment, ecosystems, and human behavior that can affect pathogens and the animals that carry them. It will be necessary to understand how all of these factors interact to accurately project future disease dynamics.

Scientists say more studies are needed to:

• Better understand how factors such as temperature and humidity affect pathogen survival and spread.

• Better understand how ecological changes and human behavior affect how infectious diseases spread.

• Model interactions between infectious diseases and climate dynamics at both larger (regional or global) and smaller (local) scales.

• Identify effective and efficient interventions to reduce risks today and as the climate continues to warm.

Finally, one of the most critical obstacles to improving our understanding of the linkages between infectious diseases and climate is a lack of good data on disease incidence for many parts of the world. These data are needed to assess the influence of climate on disease in different places, validate model projections, and track changes in disease burdens over time.

ADDITIONAL RESOURCES

• CDC – Climate Effects on Health: Diseases Carried by Vectors

• CDC – Climate Effects on Health:  Food and Waterborne Diarrheal Disease

• Climate at the National Academies

• Intergovernmental Panel on Climate Change – Climate Change 2022: Impacts, Adaptation and Vulnerability

• Modeling the Health Risks of Climate Change: Workshop Summary – National Academies of Sciences, Engineering, and Medicine (2015)

• National Academies of Sciences, Engineering, and Medicine – Earth Sciences | Topic

• National Academies of Sciences, Engineering, and Medicine – Energy and Energy Conservation | Topic

• National Academies of Sciences, Engineering, and Medicine – Environment and Environmental Studies | Topic

• National Academies of Sciences, Engineering, and Medicine – Health and Medicine | Topic

• Under the Weather: Climate, Ecosystems, and Infectious Disease – National Academies of Sciences, Engineering, and Medicine (2001)