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Rail and a stablehand fell ill, and Rail died. Investigators eventually traced the virus to flying foxes (fruit bats), which had passed it to horses. The horses served as an amplifying host between the bats and people, which showed that a spillover chain can run through a domestic animal.
A reservoir host is a species that carries a pathogen chronically, often without symptoms, and keeps it circulating between human outbreaks. Epidemics like Ebola vanish and reappear because the virus persists in an unidentified animal population between spillovers.
Pandemics are not random acts of nature. They follow from human actions that increase contact between people and wildlife: deforestation, bushmeat hunting, livestock intensification, wildlife markets and road-building into forests. Each disturbance gives animal viruses new chances to try out a human host. Rising human population and global air travel then turn a local jump into a worldwide spread, which makes a 'Next Big One' a matter of when, not if.
Quammen invites seeing Homo sapiens the way an ecologist sees an insect species in an outbreak phase. Our huge, dense and interconnected population is itself an unprecedented food source for pathogens. Outbreaks in other species, like forest tent caterpillars, typically end in a crash caused by viral disease. Viewed this way, human abundance is not just a backdrop to pandemics but the ecological condition that invites them.