The balance between humans and pathogens is undergoing a rapid, dramatic shift, and researchers believe that anthropogenic environmental influences are playing a major role. The relationship between humans and pathogens is always evolving, but the increasing effect of civilization on climate change, as well as animal intermingling, is pushing this evolutionary equilibrium out of favor. The 2003 SARS (Severe Acute Respiratory Syndrome) epidemic is a testament to this anthropogenic effect, laden with insight that could help shape our response to similar emerging infectious diseases.

Given a large enough time span, animals and parasites could theoretically reach a balance where the parasites could produce ample progeny and the hosts could live without significant detriments to their fitness. However, sometimes there is a crossover of parasitic microorganisms from one species to another, where there is no background of co-evolution. This potentiates a compromise in the new host’s physiology and for humans, a staggering economic and public health burden.

Of the approximately 8,000 people infected during the SARS epidemic of 2002-2004, 774 died. Along with these deaths came widespread fear as well as a significant financial investment in SARS therapies.1 Testing has shown that horseshoe bats in Southern China contain a significant SARS viral load genetically similar to what was found in human SARS patients.2 The bats also do not exhibit any blatant disease symptoms, which indicates some sort of adaptation to the virus.2 This leaves other previously unexposed animals living in the same habitat susceptible to viral transmission, which could be potentially followed by fatal disease. The general theory is that an increase in availability as well as consumption of exotic animals in wet markets, where all types of animals and organisms are forcibly commingled for sale, sparked a deadly SARS virus crossover into humans. Palm civets, small mammals that dwell primarily in forested areas, were a popular item in these markets and have also been suggested as the possible missing link.2 Large numbers of these civets were torn from their habitats in the wild with the increase in demand for exotic animals. Repeated testing has shown that they contain a large SARS virus load with genetics similar to the strain presumably responsible for the epidemic in humans.2 This has all culminated with the hypothesis that bat-driven civet infections in the wild translated into human infections when wet markets prompted an influx of wild civet consumption. These infections are only one case of humans forcing nature’s hand.  

In addition to the direct and dramatic manipulation of the environment by humans, there is a gradual shift in the global climate that is now starting to show its effects on the migration of whole animal populations. In the United States alone, there is already a significant change in the habitats of small mammals and waterfowl.3 Climate plays a key role in determining how livable an area is. The temperature on the earth is increasing between 0.15-0.2 degrees Celsius every decade, and the rate at which the temperature is increasing is accelerating.4 Even though this range may not sound too alarming, it is only an average. A small temperature increase results in disproportionate variability of climate.5 For instance, when a certain area experiences an increase in climate variability, it drives out species that cannot withstand the corresponding change in weather.

The accelerating change in temperature across the globe is a direct cause for animal migration and with the mass migrations of different animals comes the mass movement of different pathogens. This introduction of foreign pathogens into certain animal communities is the perfect primer for the next deadly zoonosis, like SARS. Just as in the markets in China, any sudden mixing of  foreign species creates a melting pot of pathogens — essentially, a potential hub for disease.

The SARS epidemic truly is a testament to the role of humans in the origin of diseases as well as the deadly potential of interspecies pathogen crossover. And with global warming becoming a cause for extensive mixing of animal populations, vigilance of possible pathogen movement will become necessary. At the end of the day, practices and phenomena that are cause for the intermingling of species need to be revisited and analyzed to determine whether they are ultimately allowing for the emergence of the next deadly infectious disease.


  1. “SARS Basics Fact Sheet.” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 02 July 2012. Web. 26 Mar. 2017.
  2. Lau, SK, Woo, PC, Li, KS et al. Severe acute respiratory syndrome coronavirus-like virus inChinese horseshoe bats. Proc Natl Acad Sci USA. 2005; 102: 14040–14045
  3. “Effects on Wildlife and Habitat.” National Wildlife Federation. National Wildlife Federation,n.d. Web. 26 Mar. 2017
  4. “Global Surface Temperature | NASA Global Climate Change.” NASA. NASA, 23 Feb.Web. 26 Mar. 2017.
  5. “Understanding the Link Between Climate Change and Extreme Weather.” EPA.
  6. Environmental Protection Agency, 19 Oct. 2016. Web. 26 Mar. 2017.

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