28 November 2020 | New Scientist | 45

The health of humans, wildlife and

ecosystems is intimately connected. As

our population grows, more people live in

contact with wild and domestic animals and

so the odds rise that “zoonotic” pathogens

will spill over species barriers to infect us.

Yet even if new viral pandemics that spread

from animals are inevitable, they are also

staggeringly unlikely events. Fate and biology

must combine in a precise way to put the

wrong virus in the wrong place at the wrong

time. First, a virus circulating in an animal

must pose a threat to humans. Most don’t.

They lack one or more of the bits of molecular

machinery needed to bind onto human cells,

replicate once inside and then spread further

to other cells and other people – all without

being mopped up by the immune system.

Next, the animal harbouring a potentially

dangerous virus has to come into contact

with a susceptible person and shed enough

infectious material near them to pose

a threat. Lastly, the person needs to be

infected by the virus somehow and be in

sufficient contact with others to spread it.

Breaking barriers

Thankfully, this series of hurdles is so difficult

for a virus to clear that, of the millions of

different types of viruses believed to be out

there, fewer than 300 are known to infect

humans. Unfortunately, however, some

of the natural barriers that protect us

from the viral threat are eroding and others

have been smashed to bits. International

The hunt is on

What if we could track down the virus that will

cause the next pandemic before it leaps from

other animals to humans, asks David Adam

L

URKING in the air, water, soil and inside
every other living creature, viruses
have us surrounded and hopelessly
outnumbered. For every star in the known
universe, there are at least 10 million viruses
on Earth. They are so small that more than
100 million can fit on a pinhead.
As 2020 has shown, just one of these
is enough to bring society to its knees.
The covid-19 pandemic offers a grim
demonstration of how hard it can be
to stop a new infection once it takes hold
in the human population.
But what if we could hunt down the
next pandemic-causing virus before it
starts spreading? If surveillance of viruses
evolving in animals could identify the
likely candidates, then we might be able
to pinpoint the all-important leap they
could make into humans. And by identifying
the animal species carrying the most
problematic viruses, measures could be
put in place to prevent their spread.
This kind of viral detective hunt is a
Herculean endeavour, even before you add
the difficulty of predicting which candidate
out of millions will go on to infect us.
Critics argue that it is impossible to stop
the occasional rogue virus from jumping
into humans and that we should instead
focus on stamping out those infections
when they occur. The debate has split
scientists, but it needs to be resolved soon.
Even as we continue to battle covid-19, the
clock is ticking down towards the next
outbreak of a novel pathogen. >