Researchers at the University of Edinburgh have published a catalog of human-infecting RNA viruses aimed at separating scary-sounding animal spillovers from pathogens that can plausibly start the next public health emergency.
The work, described by Mark Woolhouse, a professor of infectious disease epidemiology at the University of Edinburgh, focuses on a blunt point that gets lost whenever a new virus turns up in a patient: a virus cannot cause a pandemic unless infected people can pass it to other people. Bad disease is not enough. Novelty is not enough. Transmission is the machine.
According to Woolhouse, scientists typically identify two or three viruses each year that have not previously been seen in humans. Most do not become famous. Some barely survive in the medical literature. A few, such as HIV-1, identified in 1983, and SARS-CoV-2, identified in 2020, preceded pandemics that killed tens of millions of people, he wrote.
The Edinburgh team’s catalog covers 239 RNA virus species known to infect humans. RNA viruses matter because many major recent outbreaks have come from this group, even though researchers have identified thousands of RNA virus species and suspect millions more exist. The catalog is intended to help public health researchers judge which known human-infecting RNA viruses deserve the most attention when resources are limited, which they usually are.
Transmission beats vibes
Woolhouse said about two-thirds of the viruses in the catalog are zoonotic in the practical sense: people usually catch them from animals, and an infected person is highly unlikely to infect another person. Rabies is his example. It is lethal and terrifying, but it has not become a human-to-human epidemic engine despite tens of thousands of human cases each year, according to World Health Organization data cited by Woolhouse.
That does not mean zoonotic viruses are irrelevant. Bird flu worries scientists because viruses evolve, and a virus that already infects humans from animals could, in theory, gain better human transmission. Woolhouse’s more uncomfortable point is that there is no documented example of an RNA virus making that jump after being discovered as a zoonotic human infection.
The larger threat, in his view, comes from viruses that already spread among people. They may later become more transmissible, as SARS-CoV-2 variants did, or they may reach settings where existing transmission is enough to sustain an outbreak. Woolhouse cited Zaire ebolavirus in West Africa in 2014 as an example of a virus whose spread changed when it reached urban conditions after being associated with more remote outbreaks.
The catalog also flags viruses capable of human transmission that have so far produced only limited chains of infection. Their R number, the average number of people infected by one case, may be too low to keep spreading in ordinary conditions. Change the conditions and the math can change too.
What disease X might resemble
Woolhouse wrote that previous outbreak lists have been short but useful. Zaire ebolavirus, chikungunya, Zika, Oropouche and mpox were already on such lists before causing major epidemics, he said. He also pointed to Andes hantavirus, linked to a recent cruise ship outbreak, and Bundibugyo ebolavirus, now spreading in central Africa, as rarer names that have become less obscure.
The team’s historical work also feeds into predictions about “disease X,” the placeholder term for a future pandemic pathogen. Woolhouse said his group argued in 2019 that highly transmissible viruses tend to be close relatives of viruses that already spread between humans, while emerging separately from animals. He said SARS-CoV-2 fit that pattern, being closely related to the original SARS coronavirus while emerging independently, possibly through an indirect route from bats.
Woolhouse said Andes and Bundibugyo viruses do not have the profile of a likely global pandemic starter. A novel virus related to measles would be more alarming, he argued, because measles sits in a family of pathogens with demonstrated human transmissibility.
The practical lesson is less cinematic than “find every virus.” Woolhouse said Andes, Bundibugyo and COVID had all spread for weeks before detection. Faster discovery and interpretation of new infections would give outbreak responders less catching up to do, which is not glamorous, but pandemics are fond of administrative delay.
This story draws on original reporting from Ars Technica.
