Already, the first two months of 2024 have seen outbreaks of measles in the US and the UK, a global shortage of the cholera vaccine and even a case of bubonic plague for the unlucky owner of an infected cat in Oregon.
With what seems like a myriad of infectious diseases we thought were long gone resurfacing, it begs the question: how much disease re-emergence is normal?
To find out, we spoke to Paul Hunter, professor of medicine at UEA’s Norwich Medical School. Paul tells us why these diseases make comebacks, the terrifying truth about where the last strongholds of diseases are, and what it would take to plunge society into another Black Death.
How normal is it for infectious diseases that we think have disappeared to just come back?
The issue is whether they've actually gone or not. A lot of infections that were common once and still spread within society then come back with force in the future. Globally, we typically see one or two significant emergences or re-emergences a year on average.
For many of the classic infectious diseases that seem to re-emerge, they’ve probably been there pretty much the whole time, but re-emergence is usually more on a regional level. For example, if cholera spreads into an area where it hasn't caused problems for decades.
Is it possible to fully wipe a disease from the face of the planet?
We did it with smallpox. We thought we might be able to do that with polio, but it's not looking good. So generally, no – we don’t expect to eradicate most infections. Once the infection spreads globally and through society, you can manage and control it but it's almost impossible to eradicate.
With smallpox, the vaccine was highly effective. There was also a lot of will to get rid of it and we could diagnose it fairly easily, so you could quickly tell whether it had spread to an area.
Nowadays, there are a few problems. Many infections are infectious before somebody gets sick. As we saw with COVID, trying to eradicate an infection when people who can spread it are otherwise well is nigh impossible.
It is also difficult to do in parts of the world where there is conflict. We’ve got vaccines for Ebola now, for example, but we weren’t able to control the outbreak in the Democratic Republic of Congo as well as we would have done because of conflict.
What are some of the main ways a disease might return?
The first is that the virus might evolve. We see this with influenza, where the virus evolves over time. Sometimes you can get a big step in evolution that makes that virus then spread more rapidly; those are called ‘escaped mutations’. We saw this with all of the COVID mutations.
The other is how the disease spreads to humans. There are some diseases like plague that are there all the time in the environment: ‘sylvatic plague’ is where the plague spreads through rodents. Something might happen to the environment that suddenly brings people into contact with those animals.
For example, in the US, you see the occasional case of plague in people who are hunters or who get too close to prairie dogs thinking they look cute.
In Madagascar, a couple of decades ago, there was a big outbreak of plague, primarily because there was a lot of environmental degradation. This meant that the rats that were living close to humans were also coming into contact with other animals that were infected with the plague.
Climate change is also affecting the distribution of many infectious diseases. Diseases are spreading into areas where they've never or rarely been seen before. In Europe, along the Mediterranean coast, we're starting to see small outbreaks of dengue fever because of warming. (Dengue is not typically found in Europe, but rather in tropical regions).
The other big reason is reduced vaccination availability or uptake. That's what's driving the problems that we're seeing with measles in the UK at the moment.
I had measles when I was a child, and most people in my generation did. But if you're a lot younger, you've never seen a case of measles. So, you might think, why would you want the vaccine? But once vaccine uptake drops in a society, then measles can start to spread.
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What would it take for a re-emerging disease like the bubonic plague to get out of hand?
There has not actually been that much difference in the organism of the bacterial infection. Plague can be just as bad as it ever was, but because it's a bacteria we do have antibiotics for it, so we can treat it in a way that they didn't have in medieval times.
It can still be very nasty and even fatal, but with adequate treatment, people shouldn't be dying of the plague anymore, even if they catch it.
So for it to get out of hand, you’re looking at a collapse of society – which, to a certain extent is what was happening in Madagascar. Conflict, environmental collapse and economic collapse all coming together mean people live in closer contact with animals. So conflict areas are often places where diseases spread.
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Which would be the worst infectious disease to resurface?
Cholera is a major concern in some parts of the world, and it’s the thing that I worry about most in Ukraine and Gaza. Neither conflict area currently has cholera, as far as we’re aware, but if it did get there, it would probably spread very rapidly and cause untold numbers of deaths.
We’re currently seeing a shortage of the cholera vaccine. Partly because a lot of vaccine production efforts moved to COVID and also because people were not able to work during the pandemic. But it is also partly because of the demand because cholera infections have been going up in the last two to three years.
Typically, we store a lot of cholera vaccines globally, but the store currently has a negative value. Everything has gone, and whatever they're making in the next few weeks has already been allocated to countries where cholera is a problem.
The thing that would probably cause the most fatalities would be another big influenza pandemic like the Spanish Flu in 1917. It also tended to kill younger people, whereas COVID tended to kill older people.
Spanish Flu was a very unpleasant death: your lungs filled up with water and you effectively drowned in your own respiratory secretions. That has the potential to kill more people than most of the other infections.
Another risk is, of course, international travel – as we saw with COVID. What are some of the unwanted souvenirs we can bring back from abroad?
One of the biggest threats of overseas travel is antibiotic-resistant organisms, which some people would characterise as emerging infections. These are infections that have been around for some time before, but now we can't treat them because they're resistant to multiple drugs.
A lot of this comes from sex tourism, as we’ve seen in the past with drug-resistant gonorrhoea.
The other issue is medical tourism. That’s when people travel to other countries for cheaper plastic surgery or even dental reconstruction surgery. There is a lot of evidence that this has contributed to the spread of some of the more highly resistant organisms that we see circulating today.
Listen to the full interview on Instant Genius.
About our expert
Prof Paul Hunter researches the epidemiology of emerging infectious diseases at the University of East Anglia (UEA)'s Norwich Medical School, particularly where those diseases are linked to environmental factors. His work on this has been published in the journals PLoS One, The Lancet Public Health, and the British Medical Journal (BMJ).
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