As the 1990s progressed, it became clear to those of us in public health that a wave of infectious diseases was striking humans and many other forms of life. Humans faced multi-drug-resistant tuberculosis, Ebola, HIV, and dozens of other new pathogens. Crops were becoming infested with insects and infected with emerging viruses. Dolphins, whales, and seals were suffering from measles-like viruses, while fish were going belly up en masse with increasing frequency. Even trees were in trouble.
Throughout the 1990s, I’d been trying to forge a new synthesis that would explain how a changing world could breed a wave of epidemics. I had begun my intellectual exploration into health and global change by gazing at a diagram that hangs in frames on the walls of public health offices nationwide. The diagram consists of three interlocking circles, emblazoned with the letters A, H, and E. It is known as a Venn diagram, and it holds the key to epidemiology, the study of epidemics.
The circle labeled A represents the agent—meaning the bacterium, virus, parasite, or fungus that can, if conditions are right, infect a person, plant, or animal. The second circle, H, represents the host—the organism that becomes infected with the agent. The third circle, E, represents the environment—the external conditions that determine whether the agent will invade a host. The same three factors—agent, host, and environment—control whether people develop other diseases as well. The take-home lesson is that there are most often multiple causes for any one person’s sickness.
The agent causing tuberculosis, for example, is the bacterium Mycobacterium tuberculosis. But even if M. tuberculosis is present, the disease won’t always develop. It will be more likely to occur if the host is weakened, perhaps by malnutrition or an HIV infection, and if conditions are ripe for transmission. Ideal tuberculosis-transmitting conditions occur in close quarters, such as those in the gold mines of South Africa or 1980s-era crack houses or prisons in New York City, where infected people cough profusely into common airspace. A host with a strong immune system living in a healthy environment can usually fight off the infection by surrounding the slow-growing bacteria with immune cells, effectively quarantining them and preventing the disease. But a weak host in close quarters is more likely to be infected and have trouble fighting the disease.
This framework of agent, host, and environment, I’d realized, could be adapted to assess the impacts of global change. When global change is considered, ecosystems are the host. This analogy works on several levels. First, like the human immune system, both land-and ocean-based ecosystems have components that fight disease. In the immune system, antibodies stun invading pathogens, and white blood cells devour them. In terrestrial (land-based) ecosystems, birds of prey, like the spotted owls of the U.S. Pacific Northwest, eat rodents that can carry Lyme-disease-infected ticks, hantavirus, and bubonic plague. In marine (ocean-based) ecosystems, baleen whales and oysters filter-feed on algae and animal plankton, preventing the plankton from overgrowing into harmful algal blooms.
Second, just as a host is influenced by its environment, every ecosystem is influenced by the global environment. This includes the conditions in the lower atmosphere (troposphere), the upper atmosphere (stratosphere), the biosphere, the ice cover (cryosphere), and the world ocean. Even disregarding climate change, humans have made huge changes to the global environment. By using chlorofluorocarbons and related chemicals in our air conditioners and antiperspirants, we’ve damaged the ozone layer in the stratosphere that protects all land-based life from the sun’s damaging ultraviolet rays.
By overfishing, we’ve decimated once-abundant populations of cod and many other species. By unwittingly releasing dangerous synthetic chemicals that act like hormones in animal bodies, we’ve altered the fate of countless species. The list of disturbances goes on. Climate change portends larger changes by affecting the viability of entire ecosystems. Persistent warming can kill off vegetation, turning grasslands into deserts, as it did when a changing climate transformed the Sudanese Sahara 5,500 years ago from a semiarid grassland suitable for grazing sheep to the bone dry desert it is today.
Warming seas make it harder for coral to reproduce, contributing to the coral bleaching events that are destroying reefs worldwide. Warmer and more variable weather can enable insects, including crop pests, finally, the analogy works because human civilization is disrupting the functioning of the ecosystems that supply us with healthy food, clean air, and pure water, just as pathogenic microbes disrupt the functioning of the host’s body, upon whom they rely for life support.
Human civilizations are also disrupting the global environment. The overlapping circles of this Venn diagram represent these interactions. All of our social structures—our economy, our legal system, our energy system—influence both ecosystems and the global environment. That means that the agent in this analogy is usThis Venn diagram is useful in part because it offers an easy-to-grasp framework that illustrates how complex real-world systems work.
Conditions in a host, whether human or ecosystem, must be conducive for an agent, be it microbe or human society, to flourish. And when conditions are such that the agent, host, and environment are all disturbed, a small problem can turn into a big one. A cold virus invading the throat of an overstressed person can cause days of sickness; a wildlife disease invading a disturbed and weakened ecosystem can spread and become an epidemic.
