Why Ecologists Study Infectious Diseases And What We Can Learn About Public Health By Studying Nature

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I’m John Drake and I’m a professor at the University of Georgia. I’m a new contributor to Forbes.com. My area is infectious diseases and the environment. By training, I am an ecologist.

What does ecology have to do with infectious diseases?

Ecology is the branch of science that seeks to understand organisms in their natural context, i.e. in their environments. For instance, ecologists study how the environment affects animal behavior, plant metabolism, and every other aspect of organismal function — a branch of ecology known as physiological ecology. Ecologists also study how the abundance of a species changes over time and space — a sub-discipline known as population dynamics. Community ecologists study the interactions among different species. Ecosystem ecologists study the role organisms play in the flows of energy and elements like carbon, nitrogen, and phosphorus through nature.

In the past quarter-century there has been a flourishing of activity known as the ecology of infectious diseases. To understand me, it’s helpful to understand why ecologists are interested in infectious diseases. Understanding this suggests a few things we might learn from the study of nature for the improvement of public health.

Why did ecologists start studying infectious diseases?

To some extent, ecologists have always studied infectious diseases. In most cases, infectious diseases are caused by parasites or pathogens, although there are interesting counterexamples. (For instance, devil facial tumor disease is transmissible cancer that causes disease in Tasmanian devils, a carnivorous marsupial found on the island of Tasmania.)

The difference between “parasite” and “pathogen” is really a semantic one. Most ecologists don’t make a sharp distinction. Roughly, parasites are multicellular organisms like worms (e.g., the trematodes that cause human schistosomiasis) and arthropods (e.g., lice, mites, and fleas), while pathogens are single-celled organisms like bacteria or protozoa, or even quasi-organisms like viruses (including SARS-CoV-2, which causes COVID-19) and prions (like those that cause the “zombie” chronic wasting disease that affects deer and other cervids). Some scientists refer to the former as “macroparasites” and the latter as “microparasites”. But the point is that parasitism is an interaction between two species. That makes it a subject of ecology.

Another reason ecologists have been fascinated with infectious diseases is because they behave nonlinearly — the effect of a variable on the disease is not a simple straight line, and in fact, can be wildly irregular. Ecosystems, and the interactions of species in particular, are full of nonlinearities, so ecologists have long been fascinated with the patterns that result.

Ecology is one of the origins of chaos theory, which is based on nonlinear dynamics. In the 1980s and 1990s, when ecologists were developing methods for detecting nonlinear interactions in data, they turned to infectious diseases (especially measles) as a testbed for their new statistical techniques, partly because of the high quality of infectious disease incidence data. But ecological data are typically very noisy and require special-purpose modeling techniques. The upshot is that ecologists have developed a lot of very powerful methods for detecting and modeling nonlinear interactions in noisy data.

A third reason ecologists have studied infectious diseases is the increasingly important role of emerging infectious diseases (EIDs). EIDs are defined by the World Health Organization as diseases that either have recently appeared for the first time or have existed previously but are newly spreading rapidly.

The majority of emerging diseases are zoonotic, meaning they crossed from animal to human populations through “spillover” (transmission from an animal to a person). Ecologists are particularly well-positioned to study the process of spillover since they already study animal species in their natural habitats, and are often concerned with the aspects of human interactions with the environment that create the opportunities for spillover.

What can we learn about public health by studying nature?

Until relatively recently, public health and ecology have not had a particularly cozy relationship. Public health is concerned with the well-being of people. Ecology, to the extent that it is motivated by welfare, has typically been concerned with the welfare of non-human animals and the protection of the environment. When it comes to well-being, humans and nature often seem to be in conflict. Recently, however, there has been a surge of interest in the ways that human, animal, and environmental well-being are mutually interconnected, a concept referred to by the CDC and others as “One Health”.

Ecologists have a lot to contribute to understanding one health. For more than a century, ecologists have been working out how different species, including parasites and their hosts, interact with each other under natural conditions. Ecologists have developed sophisticated models and statistical techniques for parsing these interactions, variations of which have been useful for understanding the dynamics of human diseases ranging from measles to COVID-19. Finally, ecology, perhaps more than any other science, has sought to understand how human impacts on the environment have affected other species and also brought those other species in contact with people, a contact which has been the source of many recent emerging diseases and more likely than not will be the source of the next global pandemic.

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