Corona lessons from cholera: miasma theory vindicated over contagionism (mediated vs. person-to-person transmission)

I'm going to start corona-posting some more, but found it necessary to preface it with a look back on the history of modern epidemiology, from the 19th century, whose major concern was recurring pandemics of cholera.

The standard Reddit-tier take is that on one side was the miasma theory, which held that foul outdoor air contaminated by decaying organic matter was to blame for giving people cholera. Boo, hiss. On the other side -- oh hell yeah -- was the germ theory, which was eventually proven correct.

In reality those two theories are completely orthogonal to each other, and one neither discredits nor testifies to the other.

The germ theory is about causative agents, such as bacteria, viruses, and other pathogens -- as opposed to other things that could harm a person, like toxins, allergens, nutritional deficiency, and so on.

The miasma theory was about how a causative agent -- regardless of whatever it happened to be -- circulated throughout a population, spreading disease. Miasma theory relied on a medium (such as contaminated air) to transmit the causative agent from one person to another, indirectly. Its rival, contagion theory, held that transmission was through unmediated contact in close proximity (such as coughing in someone else's face).

To simplify things, we can talk about an "encounter" between a sick person and a healthy person. Miasma theory does not require such an encounter for transmission to occur, whereas contagion theory does. Miasma requires a medium, contagion does not.

We will get into how this changes a formal mathematical model of epidemic dynamics in a later post. But suffice it to say for now that the contagion model is the standard S-I-R model, tracking susceptible, infected, and recovered individuals, who have encounters with each other. Miasma requires a new variable to track, namely the concentration of the contaminant in the medium, as well as descriptions of how infected and susceptible individuals come into contact with that medium (their encounters with each other being irrelevant).

This is the actual substance of the actual debate that raged during an actual pandemic of an actual disease. Only clueless autists would ignore the historical context of the debate when trying to identify what the crux of the matter was, and what was incidental or irrelevant. It was about whether, for cholera in 19th-C. Europe, transmission was mediated or direct. That's all -- every other detail is secondary, tertiary, and further removed from the fundamental disagreement between the contagion vs. miasma camps.

Specifically, there is nothing relevant about air being the medium for the miasma camp -- air is only one of many potential media via which a causative agent could be transmitted indirectly from one person to another. Even if you assumed it was air, it's irrelevant that they thought it was outdoor air -- indoor air is still a medium for transmission. And it's irrelevant that they highlighted air of a foul odor -- neutral or pleasant-smelling air is still a medium for transmission.

And again, the causative agent question is totally independent -- miasma theory does not depend on decaying organic matter / vapors it gave off as the causative agent. That just happened to be the source they believed in. They could have believed in germs or bacteria in particular as the causative agent, and that would not have changed the nature of their disagreement with the contagionists, over whether that agent was transmitted through a medium or directly.

So in evaluating who won the debate, we look at the level where they actually disagreed, rather than consider every aspect of the theory together. The two camps disagreed at a very fundamental level, over direct vs. mediated transmission -- so we ignore any details that pertain to finer levels of specificity.

For example, it would take two miasma theorists to debate over whether it was air or some other medium, whether the air responsible came from outdoors or indoors, whether the air had to be foul or neutral-smelling, etc. That was not the debate, so we ignore those details of the theory in evaluating its performance against its rival, which denied that mediated transmission of any kind was taking place.

As it turns out, the miasma theory was decisively vindicated over the contagion theory. Cholera was transmitted through a contaminated material medium, whereby the initial sick person had no encounter whatsoever with the healthy people he would eventually infect.

At finer levels of specificity, the miasma proponents got the picture wrong. It was contaminated water, not air, that was the medium. And the initial contamination came not from outdoors, but indoors (a sick person's diarrhea, which then entered the sewers). They were right about the contaminated medium smelling foul, though.

But those details were not what they and their rivals were arguing over, so we ignore them in deciding which side won. The contagionists held that cholera was transmitted by a direct process similar to coughing in someone's face, or exchanging bodily fluids during sex. They were dead wrong -- cholera is a canonical example of mediated transmission, where the sick person and healthy person were not in the same room, building, or perhaps even block or neighborhood at the same time.

* * *

The victory of the miasmists doesn't end at the level of pure understanding, though, but continues through to the applied / solutions level. If the contagionists had been correct, then some kind of isolation of a sick person would have had a major effect in breaking the chain of transmission. But if transmission is through a medium, then isolating people from people will not necessarily break transmission at all -- if the medium is left to connect people indirectly.

And sure enough, quarantines and keeping everybody in their own little homes did not stop cholera, since people were connected by a public water supply. Isolated sick people continued to excrete their waste into that water supply, and healthy people continued to come into contact with that water supply through pumps (for drinking, washing, etc.).

Notice that the miasmists were not autists who said, "OK then, just sever the links to the medium". That is the analogy to breaking the chain of transmission in the contagion model. Keep sick people from excreting into the water supply -- how? Tell every sick person, of whom there are legions during a pandemic, to build their own water supply? Or maybe tell the healthy people not to drink or wash from the water supply when an outbreak is taking place -- so what's the point of supplying the water then? And water is a necessity, not a luxury.

These links to the medium cannot be severed, and "isolation" from the contaminated medium was not the solution the miasmists hit on.

Rather, it was sanitation -- i.e., purifying the medium of the contaminants responsible for the disease (whether or not this process cleared out other contaminants, though hopefully it would). Maybe they would remain present in the medium, but either killed or neutralized of their disease-causing power. Maybe they could be removed from the medium altogether, by a filter or something. Maybe the medium could be replenished so quickly with fresh material that the concentration of the contaminant could never reach a high enough level to pose a serious risk to someone who came into contact with it.

In the case of contaminated water, they could at least separate the supplies of water carrying away waste vs. bringing water in for consumption. That seems to sever one of the links -- while the sick are still polluting a medium, the healthy do not consume from that medium, but from a separate source.

However this does not generalize to all media. Air, for example, cannot be channeled into two decoupled paths for bringing-in vs. taking-away. The air you exhale into, is going to be breathed in by someone else. Like water, air is a necessity, so there's no isolation strategy that will work, only sanitation.

Insect-borne diseases behave much like a miasma disease, where transmission is mediated by the insect, which bites a sick host and then bites a healthy target. The source and destination have no encounter, and could be miles away from each other at all times. Isolating one person from another would not stop the spread, only neutralizing the insect, isolating from the insect, etc. Unlike the above miasma diseases, contact with this medium is not a necessity -- you never want to get bitten by them. But then miasma theory never said that the medium was a necessity, only that there was such a medium rather than transmission through encounters between sick and healthy individuals.

And as it happens, the diseases which motivated miasma theory to begin with were all ones with mediated transmission -- bubonic plague (fleas), malaria (Italian for "bad air," mediated by mosquitoes, though), and cholera (water). Air as a contaminated medium is an even greater vindication of the theory, since it gets the medium correct as well -- but that will have to wait until discussion of the SARS-like diseases, and perhaps respiratory diseases more broadly.

The point is, miasma theorists were smart, not stupid -- they could tell their disease of interest wasn't transmitted by encounters between sick and healthy, but the particular medium was hard to discern. Fleas? Mosquitoes? Water? Air? I dunno, some shit like that, though -- we can tell that it does not spread through one person encountering another, and that isolating people does not stop the spread.

Contagion theory does apply to some diseases, although given how much the picture changes when you view air as a potentially contaminated medium, I'm putting all respiratory diseases on hold as examples of the contagion model. Sure, someone could cough in another person's face, but they could just as easily cough into an empty room, where the particles remain suspended as aerosols, and someone else comes in minutes or hours after the sick person has left the room altogether. That's mediated, not an encounter.

The only uncontroversial examples of the contagion model are sexually transmitted diseases -- can't pass those along without a direct-contact encounter between two people. Ditto for ones passed from a pregnant mother to her baby, non-genetically. But these are usually clear in their mode of transmission, so it doesn't take an insightful person to describe them with the contagion model.

It's clearly too late to reclaim the title of "miasma" theory non-ironically, due to the autistic rewriting and mystifying of what took place during the 19th-C. cholera pandemics in Europe. But I think "mediated transmission" is still OK, as long as it's clearly opposed to "contagion" or "person-to-person" models.