Over the past century, many notable viruses have emerged from animals to cause widespread illness and death in people. The list includes the pathogens behind pandemic influenza, Ebola, Zika, West Nile fever, SARS and now COVID, brought on by the virus SARS-CoV-2. For all of these microbes, the animal species that served as the original source of spillover was hard to find. And for many, that source still has not been conclusively identified. Confirming the circumstances and key participants involved in the early emergence of an infectious disease is a holy grail of this type of scientific inquiry: difficult to track and even more difficult to prove.
In ideal conditions, the first human cases involved in a zoonotic disease spillover (when a pathogen jumps from animals to humans) are reported in connection to animals present at the time of the event. This happens when the cluster of cases is large enough to be investigated and reported. But it is not necessarily the first time spillover occurred. Most spillovers are limited to more narrow animal-to-human cases. Once pathogens start to spread by human-to-human transmission, the tracks leading back to the initial animal source grow faint and become nearly impossible to follow.
Thus, animal sources for viruses that cause pandemics often remain shrouded in mystery. For some viruses, animal sources have been implicated after years or decades of large-scale international investigations. For other viruses, animal sources are highly suspected, but enough evidence has yet to be produced to pinpoint an exact species or range of species. Typically, lines of evidence are drawn over time through a trove of peer-reviewed publications, each building on the research that came before it, using more precise methods to narrow the field of possible sources. The scientific process is naturally self-correcting. Often seemingly contradictory hypotheses can initially flood the field, especially for high-impact outbreaks. But eventually, some of them are ruled out, and lines of investigation are narrowed.