Filoviruses are structurally and functionally alien among pathogens

Preston frames filoviruses — the family that includes Ebola and Marburg — as visually and biologically distinct from most viruses that infect humans: under an electron microscope they appear as long, thread-like or looped filaments rather than the more familiar spherical or geometric shapes of viruses like influenza or HIV. This structure isn't just cosmetic; these viruses attack blood vessel linings and connective tissue with unusual aggression, contributing to the hemorrhaging associated with severe cases.

What makes them especially frightening in Preston's account is the combination of high case-fatality rates in some strains with poorly understood natural reservoirs, meaning scientists couldn't reliably predict where or when the next spillover into humans or other primates would occur. The specific animal reservoir for Ebola remained scientifically uncertain for years after the events described in the book, which Preston treats as itself alarming: you can't guard against a threat whose hiding place you haven't found.

Takeaway: a pathogen's danger comes not just from lethality but from how much remains unknown about its origins and transmission.