How do we trace back disease outbreaks?

How do we track outbreaks back to their source?
22 November 2022


A map of the world showing hotspots of an infection.



How do we trace back viral outbreaks to their source? Is it just a case of tracking down patients who've got the symptoms and then finding the first one? Or is there more involved than that?


Jonathan - Well, I think it's not just viral outbreaks. I think we can look at epidemics and pandemics as one category, and perhaps it helps if we go back to the first epidemiologist, John Snow. So in about the 1830s, Europe was hit by cholera for the first time. And people didn't know what was causing this, this horrible disease. Some people thought it was due to my miasma.

Chris - Bad air.

Jonathan - Bad air, literally bad air. Other people thought that it might have been some kind of plot by the rich to poison the poor because it was poor people living crowded in sanitary urban slums that were being most affected. And John Snow had a hunch that this was a waterborne disease, which was revolutionary at the time because germ theory hadn't been accepted at all. And this made sense, or it makes sense in hindsight because for example, London, where John Snow lived, was a city of about 3 million people in the middle of the 19th century, but it had no kind of integrated sewage and water network. Enormous amounts of human waste just flowed into the Thames. And London inhabitants got their drinking water from that very same river. So when a cholera epidemic broke out in 1854, Snow ran to Soho in the center of London where the outbreak was occurring. And he undertook what's probably the first epidemiological study. He went around and he interviewed people that were affected and he tried to work out what linked them together in their working life and in their kind of everyday life. And he realized that it must be related to this water pump on Broad Street, which is still in Soho if you happen to happen to go there.

Chris - Amazing insight to think that he, because we take this for granted these days, don't we? That something must cause something. There must be a connection between all the individuals. We ask all the people what their common exposure is, and that must be the reason that they've got the problem. But for him to do that in that era where we didn't even know that germs existed, and that therefore they could be contained in a water supply and transmitted via water, that was really a big leap.

Jonathan - No, it was absolutely revolutionary. And sadly, his ideas weren't accepted in his lifetime. So he died about four years later and there was a 33 word obituary in the Lancet, the prestigious medical journal. And it didn't mention it.

Chris - But did they take him seriously at the time? So when he said, 'look, there is this water pump, there are all these cases that seem to be connected to it'. Did people take it seriously at the time, stop using the pump and the disease went away, thus proving he must have been right?

Jonathan - Yes and no. So the local authorities, they looked at his evidence and agreed to remove the handle of the pump, and then the outbreak stopped. But the broader scientific community, in particular the commissioners that were charged with looking into the causes, looking at the causes of the 54 outbreak, rejected his idea. And they kind of doubled down on the miasma theory and in this obituary to Snow a few years later, there was no mention whatsoever of his work on epidemiology. It was all about his work on anaesthetics, which was the other thing that he was known for. And it wasn't until the next outbreak, which occurred in 1866, so this was after the sewers had been built in London and the last remaining place in London to not have sewers connected, around Bethnal Green, had the big outbreak. And that finally convinced people that this was the waterborne disease. But to come back to the question, if we look at Covid now, and the way that that's traced back to the wet market in Wuhan, it's pretty similar. Epidemiologists have worked out that a large number of people that had the disease early on had been at the market. And so that's a very likely point at which it jumped over from animals to humans. But still, we're not a hundred percent certain that that's the case. It's just this circumstantial evidence.

Chris - It's interesting that there is that historical route to this aspect of epidemiology, but like many amazing leaps in science, it was too early, too soon and people didn't take it seriously and it took more convincing. I suppose it's kind of good in some respects that science is robust enough to defend itself against arguments and wants to be convinced, but we mustn't be too dogmatic in our thinking. We must be open to new arguments and new evidence, I suppose is the moral of that story.

Jonathan - I think that's a fair point. Although I'd like to think hopefully science has advanced quite a lot since the 1850s. The state of medical understanding then was pretty rudimentary and it hadn't changed much since the ancient Greeks, to be honest. So there's been a massive revolution in understanding.

Chris - Although I mean you make this point that germ theory, the idea that there were germs that could be transmitted between individuals, had yet to be born at the time that all this was going on. And I was having this same conversation with someone the other day where we were pointing out that in fact, if you look at the mortality rates and how long people were living and so on in that period in London, they were plummeting well before we even discovered what germs were. Or for instance, deaths from tuberculosis were plummeting well before we even knew what caused it. And it was all about cleaning up London, giving people better living conditions, giving people fresh water, sewage, better food. So public health did all that well before modern medicine came along.

Jonathan - Exactly. And I think it shows the importance of politics to public health. The fact that there were political reforms in the UK and in the 1860s, 1870s, that made it politically viable to basically borrow money and spend it on public health because the electorate had been expanded so much that the people that were benefiting could actually vote for that, but didn't have to pay for it.

Chris - Sounds nice. And many governments are accused of eventually running out of other people's money to spend and that's when they get voted out. Thanks for that, Jonathan.


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