Zoonotic spillover is the origin story of most pandemics

Quammen builds the book around a single unifying fact: the great majority of new infectious diseases affecting humans, including HIV, Ebola, SARS, and many strains of influenza, did not originate in humans at all. They lived quietly in animal populations — bats, primates, birds, rodents — often causing little or no illness in those natural hosts, until a specific contact event let the pathogen cross into a human body for the first time.

He uses "spillover" precisely to describe this jump, distinguishing it from the subsequent chain of human-to-human transmission that turns a single crossing event into an outbreak or pandemic. The crossing itself is usually a low-probability, almost accidental event — a hunter butchering an infected animal, a farmer exposed to bat droppings — but multiplied across billions of human-animal contacts worldwide, low probability events become statistically inevitable.

Takeaway: pandemics don't appear from nowhere — they cross a specific, traceable bridge from animal ecosystems into human ones.