Spillover: Animal Infections and the Next Human Pandemic

This is an animal sometimes mistaken for the reservoir host. Amplifiers are “especially hospitable” (36) to a virus that they often catch from the reservoir. Amplifiers produce enough virus on their own, in massive numbers, that they frequently infect another species. Horses are an “amplifier host” for Hendra, able to infect humans where bats are not. Amplifiers played a significant role in the SARS outbreak, since that virus also spread from bats via another animal: civet cats. 

These are safety designations for medical research laboratories, referring to the “biosafety level” of each. BSL-4, pioneered by scientist Karl Johnson, involves the most elaborate containment procedures, which Quammen describes as “multiple seals, negative air pressure, elaborate filters, and lab personnel working in space suits—a containment zone in which Ebola virus could be handled without risk (74). BSL-3 laboratories are for diseases that are not only harmful and highly contagious but also treatable, like plague. 

This situation describes the status of a virus at the end of an outbreak, from the perspective of the virus. If a spillover is unsuccessful—leading to relatively few new infections and ultimately ending the number of cases—that host is a dead-end host. This describes only that particular case, as Quammen highlights: “Not the virus in toto throughout its range, of course, but that lineage of virus, the one that has spilled over, betting everything on this gambit—it’s gone, caput” (83).