In a nutshell, miasma theory argued that illness was transmitted through the air, which could be poisoned by emissions from rotting organic matter. If you breathed this poisoned air, the theory said, you got sick. "Bad" air could be distinguished from "good" air by its offensive odor -- the stronger the odor, the worse the miasma. The way you prevented disease was by removing things that smelled bad to somewhere people weren't.

The miasma theory fell short compared to germ theory in several respects. While germs can be transmitted through air, for instance, they can also be transmitted through water, physical contact, etc. -- so miasma theory was spectacularly unhelpful in dealing with diseases like cholera, which is transmitted through water. There are also lots of situations where germs can be present even though there's no obvious bad smell, so miasma theory is no help there. Germ theory can explain why a doctor who doesn't sanitize his instruments between operations can pass an infection from one patient to another, for example; miasma theory cannot. And since it identifies the germ as the underlying vehicle of transmission, germ theory provides a means of protection -- kill the germ and you prevent the transmission. Miasma theory had no explanation for what exactly in the supposed miasma was causing the transmission, so its prophylactic utility was limited.

It's one of those things where people very early on made an intuitive observation (that being in places full of things that smelled bad tended to make people sick), and since that observation was "good enough" to make it useful in a lot of cases, it became part of the conventional wisdom. But once the scientific approach started finally being applied to medicine, its limitations became clear.

I dislike this idea of calling theories that ended up being wrong as "pseudo-science". Not all wrong ideas were just random ramblings. Miasma Theory came from imperfect knowledge and not understanding the mechanisms of some of their observations.

If you read the debates on how miasma works, you can totally see reasonable rational arguments if you strip away a few generations of scientific knowledge.

I agree. It becomes pseudoscience when it contradicts existing scientific knowledge without any rationale or explicitly comes from a a rejection of the scientific method. There's a difference between being pseudoscientific and being wrong. A lot of hypothesis about how the world works today, including some seemingly strong beliefs backed by pots of evidence, will be found to be wrong or incomplete as we move forward and develop better tools and science.

My understanding is that the term 'pseudo-science' refers to theories that people pretend have scientific backing but really don't, as opposed to theories that just eventually prove to be false. So I might claim that homeopathy is based on the principle of 'like-heals-like' and claim that it is based in science, but in reality there is no scientific evidence to back that idea and so homeopathy would then be pseudo-science.

Miasma theory was the belief that it was the smells themselves that caused the disease, or rather, that bad air which was synonymous with with bad smell, that caused disease. The plague doctor outfits, for example, were not actually airtight. The 'beak' contained incenses and perfumes intended to protect against miasma by preventing smelling of bad smells. It actually was not very effective (fleas getting in through under the coat was among the many defects,) and the mortality rates of plague doctors was very high.

I believe the "miasma" model was the dominant theory at the time of John Snow's famous cholera investigation. In fact, IIRC the dot map method that Snow is famous for (e.g. via Edward Tufte) had also been usedby miasma-proponents; I think the takeaway is that Snow strongest argument weren't the maps, but the investigative surveying and research he conducted in addition to those maps.

https://www.ph.ucla.edu/epi/snow/choleratheories.html

Basically it's https://en.wikipedia.org/wiki/Spontaneous_generation for disease.

Some of the mitigations would've been effective, but the mechanism was all wrong. It doesn't permit person-to-person transmission, so you could be in a place with "good air" and still be transmitting disease while thinking you can't.