What is an emerging infection?

As if anyone needed reminding, for the last 2 years the world has been in thrall to the Covid-19 pandemic. Before that, a huge outbreak of Zika virus that was linked to birth defects hit Central and South America and then rapidly spread to other countries. A couple of years before that happened, the world saw the first cases of Ebola outside of Africa and the largest recorded outbreak of the disease within the continent itself. Shortly before that, swine flu swept the globe, hot on the heels of the original SARS virus that emerged from China in late 2002 and spread internationally...

Jonathan - An emerging infection, Chris, is an infection that has suddenly appeared within a population or sometimes we can think of emerging infections as something that has been infecting humans for quite some time, but it suddenly gets introduced into a new geographical range, for example. So generally we think of them as new kids on the block as it were.

Chris - And where do most of them come from? When we've got a new entity, it didn't just pop into existence. It must have come from somewhere. So where do most of them appear from?

Jonathan - It's a great question. If we think about the novel infections, the unknown infections, for example, things like SARS Coronavirus, MERS coronavirus, and indeed the recent SARS two. These are viruses which have been circulating in animals, we think in bats, and they may well find their way into what we call an intermediate species. This is a species that acts as a kind of gateway into humans. So the virus jumps from the bats potentially into this intermediate species and then into humans. And if the virus is able to infect and replicate and then transmit in humans, that virus can then start to explore humans as a new host. Now, sometimes the virus needs to gain some mutations to be able to do that, but, sometimes the virus is pretty much ready to go and we think that's certainly the case for the related SARS viruses. And so that's one example of a virus popping out of an animal host into humans. And we can think of HIV as being very similar, also Ebola virus. But if we think about things like a Zika, for example, what we see there is a virus that has expanded its range. So it's gone to new geographical locations. And so what's happened is the virus has been circulating for a very long time in Africa, also in the equatorial ranges within Asia. And then something's happened that has allowed that virus to then transmit across the Pacific ocean and then find its way to Central and South American shores. And so it's these sorts of activities, also things like climate change, etc., that allow the viruses to expand their geographical range in their hosts. As the climate, the globe, warms, so the insects that carry many of these infections, infections like Chikungunya virus, Dengue virus, these are all viruses that rely on insects to spread them. And as the global temperatures warm, the insect ranges increase and therefore you get the viruses introduced into populations.

Chris - So it sounds to me really that you're saying there are possibly three things going on: one is that we, in some way, bring ourselves into contact with where a virus naturally is, or we move the virus to somewhere else or, something else changes which encourages an animal or a thing that has the virus to move. But, either way, it ends up with the virus, or where it is, rubbing up against us. And that gives it the chance to jump.

Jonathan - Yeah, absolutely. If you think that human populations have increased significantly also because of that, we then need more resources. We've started to encroach on wildlife habitats. What that does is bring us into close contact with animals that we wouldn't have normally come into contact with. And, of course not just us, but also the animals that we've domesticated and tamed, for example, livestock. So it's this encroachment upon habitats, but also the globalization. So the fact that we can travel huge distances, and the fact that we ship goods over huge distances, has given not only the viruses, but sometimes the insects that the viruses rely on, the opportunity to explore new domains and new realms and new human populations.

Chris - Yes, I did read the headline when the World Cup came to South America, and they were saying "fever pitch" because of the worry about people moving on mass by the million to go and watch World Cup games. But I suppose it's that mass movement that has the potential just very briefly to amplify these sorts of outbreaks.

Jonathan - It is. You know, wherever you've got people moving over large distances very rapidly, then you can see the transfer of infectious diseases following those mass migrations. So we live in a different world than we did several decades ago. And that's why we're seeing far more of these emerging infections.