The book, written in narrative form, tells through the personal experiences of the author, who interviewed numerous pathologists and virologists globally to trace the evolution of some of the major pathogens that have affected the human species following a species leap (spillover), a natural process by which an animal pathogen evolves and becomes able to infect, reproduce and transmit within the human species, in a process called zoonosis.

Among the human activities, the author identifies some criticisms that increasingly favor the spread of epidemics, including deforestation and the destruction of natural habitats that increase contacts between wild animal species and man, pollution, the overpopulation of some areas that brings millions of people into contact in relatively very confined spaces, the possibility of ever faster and cheaper air travel that favor the possibility of spreading diseases in distant places, and the intensive in contact with billions of animals with the consequent risk of animal epidemics that can be transmitted to humans.

“People and gorillas, horses and duikers and pigs, monkeys and chimps and bats and viruses,” Quammen writes.

Kirkus awarded a star review, praising the accounts on "the thrill of the chase and the derring-do of field research", and described it as "wonderful and eye-opening.

"[1] Alice Roberts from The Guardian also recommended the book, and praised the recounts on the viruses, the "fascinating" historical details, and the combination of field research with virology, epidemiology and genetics[2] Sonia Shah from The New York Times called it "meaty" and "sprawling", complimenting the descriptions of field trips, and considered the author to be a "cheeky and incisive chronicler of the scientific method".