" Emergence and spillover are distinct concepts but interconnected. “Spillover” is the term used by disease ecologists (it has a different use for economists) to denote the moment when a pathogen passes from members of one species, as host, into members of another. It’s a focused event. Hendra virus spilled over into Drama Series (from bats) and then into Vic Rail (from horses) in September 1994. Emergence is a process, a trend. "

― David Quammen , Spillover: Animal Infections and the Next Human Pandemic