What’s Causing The Sickness?

Something bad’s gone down. Your community is getting by, because Hey, you were prepared and knew how to do some things. But now people are getting sick. There’s no word from the outside on what it might be; so you’ll have to figure it out yourselves so you can take appropriate measures. How do you discover the source of the sickness?

Or maybe it’s animals that are getting the sickness. It could be your food plants instead. That makes this problem a common one. It’s also a problem that not every physician, nurse, veterinarian, or agronomist is trained to deal with. That raises the probability that you’ll have to figure it out yourself.

People have been pretty terrible at this, historically. Sickness has been attributed to everything from immorality to witches to bad smells to planetary alignments. We’re bad at it because it takes some specific mental approaches that don’t come naturally. A good way to learn the approaches is to look at one of the early real successes in tracking down a cause of illness

The 1854 cholera epidemic in London

Cholera epidemics were common in the 1800s in many places around the world, including the largest and most prosperous cities. London was no exception and had had many outbreaks. The 1854 outbreak only lasted about 10 days, but more than 600 people died, mostly from a single neighborhood in Soho. 

So why are we looking at this particular epidemic as a guide? Because John Snow, a prominent physician, lived in Soho and did some great work in sorting out the cause of the sickness. There are some key points to his method.

Look for commonalities among the sick

Ok, this important starting point does come naturally to most of us. We look at who are affected, and try to see what they have in common. It’s likely that one of the things in common is the source of the problem.

In the case of the 1854 cholera epidemic, most of the the afflicted lived within a half mile radius of Broad Street.

So what was special about Broad Street? It had a lot of working poor, but some nicer apartments (like John Snow’s) near the edge of the affected area. It stank to high heaven — par for the course in 1854 London. Sanitation stank (Yeah I did that on purpose). It was served by a communal water pump renowned for its good taste. And of course, it was a geographically small area, so there were lots of potential personal connections.

Who’s getting sick? Some commonalities, and what they might mean

The least helpful finding is that the people getting sick are mostly the very young, very old, malnourished, or people with chronic illnesses. These are the folks with the worst immune systems, so if everyone’s getting exposed, these people are most likely to get sick.

On the other hand, if the very young are getting it but not the old or chronically ill, it’s more likely to be an infectious disease that recurs periodically. People exposed (or vaccinated) earlier in life have better immune defenses when the germ returns, so it’s mostly the wee people who haven’t seen it before that get sick.

Mostly you look for what’s there, and get meaning from that. Is it chicken farmers who are getting sick first? It might be a bird flu being caught from the chickens. Is it the patrons of a single restaurant? Look at their food sources and handling.

Look for sick people outside the common pattern

Surprise cases may be especially valuable clues. That’s because many of the sick may have lots of things in common. They may work and eat in similar places, breathe the same air, get the same water. The outlier cases may have only one or two things in common with the rest of the sick, which makes those one or two things the likely culprit.

For example, almost everyone who got sick in the 1854 cholera epidemic lived close to broad street. There were, however, two cases a couple of miles away. They were in the same household; an elderly woman and her visiting niece. So what was their connection to the Soho community? Well, the woman’s son brought her water from the Broad Street pump, renowned for its good taste.

That made either the water from the pump, or direct contact with the son very likely suspects. The son didn’t get sick, but often people’s immune systems will keep them healthy despite their being exposed, or even carriers.

Who isn’t getting the sickness? Look for counterexamples

You’ll often find several connections among many of the sick. Now comes the less intuitive part; but it’s key: Look for people that share most of the connections but aren’t sick.

In that small SoHo neighborhood, for example, there were two surprisingly sickness-free groups. Nobody who worked in the local brewery got ill. The local workhouse had only five cases. That was an oddly low number for a high population of mostly old, chronically ill, and malnourished people (prime targets, in other words).

So what was different about the people who didn’t get ill? They breathed the same air. That’s important, because the current theory was that cholera was spread by miasma (bad air), and Broad Street sure had that. They all had some personal contact with people who did get sick, so it wasn’t that.

What they didn’t do was share the water. The workhouse had its own pump. The brewery workers didn’t use the local water because they got paid, in part, in ‘small beer’. That’s a low-alcohol product that the brewers used as their main beverage (and brewing destroys microbes).

One quirk to look for here is military service. If military people, current or past, are less likely to get sick, probability rises that the source of sickness is a foreign microbe. Military people have more vaccinations and a more diverse exposure, so are better protected against foreign microbes than your average resident.

Drawing conclusions

People had looked for the source of cholera before, and mostly concluded it was from miasma. Considering how much the stink of bad sanitation was correlated with the real cause of cholera outbreaks (sewage-contaminated water), that wasn’t a terrible idea. They were led astray because they’d stopped at ‘looking for commonalities’ and not included the ‘pay attention to outliers’ and ‘look for counterexamples’ steps.

Seeing what your victims have in common can let you develop hypotheses about what’s going wrong. The natural human thing at this stage is to fall in love with your idea, look for confirming evidence, and be satisfied if it’s there. But if you stop there, you’ll be wrong a lot. 

Instead, keep in mind there are other hypotheses, and your first one may be wrong. It’s ok; as Einstein reportedly said “If we Knew what we were doing, it wouldn’t be Research!” Look for the outliers, and see what their connections to the other sick people might be. Also look for the counterexamples, and see where they fail to intersect with the sick people.

Knowledge of cause is the power to stop the sickness

Once you have a better idea of causation, you are set to start doing something effective about it. Famously, John Snow presented his conclusions to the local health officials and they removed the handle from the contaminated water pump (probably too late to do much good, by the way). But Snow’s work over the coming months persuaded many people. Better sewage handling was made a priority, and the cholera epidemics stopped.

If you don’t know the real cause, you can do all the wrong things. The whole medieval thing about killing cats because they were ‘of the devil’ in order to get rid of plague? Not so helpful.

With gratitude to Harvard University, for their free MOOC Prediction X: John Snow and the cholera outbreak of 1854., offered through EdX.