In the past decade, various viruses such as mad cow disease, bird flu, and swine flu have frightened humans around the world. Viruses such as these are called zoonotic diseases and is what Quammen suggests will cause the next great human pandemic. His website introduces this idea as:
"The next big and murderous human pandemic, the one that kills us in millions, will be caused by a new disease-new to humans, anyway. The bug that's responsible will be strange, unfamiliar, but it won't come from outer space. Odds are that the killer pathogen-most likely a virus-will spill over into humans from a nonhuman animal."
As a writer for National Geographic, Quammen was able to frequently travel to jungles, deserts, and swamps. It was in these various locations that Quammen noticed how often viruses jump from animals to humans. He use, "the science, the history, the mystery, and the human anguish as page-turning drama" to describe various zoonotic diseases such as AIDs, Ebola, SARS, and Lyme disease.
While the majority of this new book seems slightly terrifying, readers can find a bright side. In an interview with Grist.org, Quammen illuminated how this book can be a fight for biodiversity. Many scientists and conservationists insist that we should stop deforestation in the rainforests in order to search for medicines in the foliage. Quammen agrees that we should cease deforestation, but his argument is based on the zoonotic diseases. The closer we move into forests with high animal and plant diversity, the greater viral diversity we find. Although many of these viruses won't affect humans, others will. This book also reminds people that humans cannot completely control nature. All of our actions have consequences and diseases are a few of these consequences. So although this book may be a bit frightening, it does bring promises as well. While apathy may be more apparent in relation to climate change and other sustainability efforts, Quammen thinks these diseases will help people get interested more quickly. He states, "With [infectious diseases], we’re saying to people, you should do this, or you’re gonna die next week. That’s a little bit easier case to sell."