A catalog of RNA viruses, meaning viruses whose genetic code is stored in ribonucleic acid rather than DNA, now ranks the 239 species known to infect humans by their potential to cause a public health emergency. The catalog was assembled by a team led by Mark Woolhouse, a professor of infectious disease epidemiology at the University of Edinburgh, to help scientists decide immediately how seriously to treat the next unknown pathogen found in a patient. Scientists typically discover two or three never-before-seen human viruses each year; the catalog gives them a framework for sorting the dangerous from the forgettable.
Why transmissibility, not severity, is the dividing line
The catalog separates viruses by one decisive variable: whether they can pass between people. For two-thirds of the 239 listed viruses, an infected person is unlikely to transmit to anyone else. These are zoonotic viruses, a term for pathogens people catch from animals rather than from other people. Rabies is the clearest example.
That sounds reassuring. Viruses evolve quickly, and there is standing concern that a zoonotic virus might acquire the ability to spread between humans. The Edinburgh team notes there is no documented case of an RNA virus making that crossing. Rabies has not managed it despite tens of thousands of human cases each year.
The sharper risk sits with viruses that already spread person to person but have not reached scale. Their R number, the average count of additional people one infected person goes on to infect, stays low enough that chains of infection eventually die out on their own. R numbers can change. When Zaire ebolavirus moved from remote villages into west African cities in 2014, that is exactly what happened.
A short track record that turned out to be accurate
The outbreak section of the catalog has a small but exact history. Zaire ebolavirus, Chikungunya, Zika, Oropouche, and mpox were all listed before each went on to cause major epidemics. Andes hantavirus, responsible for a recent outbreak aboard a cruise ship, and Bundibugyo ebolavirus, currently spreading in central Africa, also appear on the list.
The Edinburgh team's 2019 research found that highly transmissible viruses tend to be closely related to other human-spreading viruses yet emerge from animals independently. That turned out to describe SARS-CoV-2 precisely: very similar to the original SARS coronavirus but acquired separately, perhaps indirectly from bats. The World Health Organization had flagged a SARS-like coronavirus as a disease X candidate, meaning a pathogen with pandemic potential that has no name yet, the year before COVID-19 emerged.
The detection gap that cost weeks
Neither Andes nor Bundibugyo carries the profile for a global pandemic, Woolhouse notes. But both were spreading for weeks before anyone identified them. So was COVID-19. A novel virus related to measles would carry a very different profile, and a far more serious one.