Medicine today operates under the germ theory of disease, which simply means that we understand diseases to be caused by microorganisms such as bacteria, viruses, and fungi, to name a few. The germ theory of disease started to become mainstream in the 19th century, when it was pioneered by biologists such as Louis Pasteur (1822-1895) as well as medical professionals such as Joseph Lister (1827-1912), surgeon at the Glasgow Royal Infirmary, who put Pasteur’s findings into practice by sterilising his surgical instruments and demonstrated that sterile surgery resulted in fewer patient deaths.

Before the germ theory of medicine became fully understood and accepted, however, people had other ways of explaining what caused epidemics. Over the previous three centuries (1500-1800), Europe suffered from multiple outbreaks of infectious diseases, including cholera and smallpox, as well as the notorious bubonic plague. Throughout this whole period, the medical field was dominated by the miasma theory. Miasma comes from Greek and means pollution, and that is literally what doctors believed infectious diseases to be caused by: bad air. This “bad air” was believed caused by noxious vapours and smells that arise from rotting organic matter which could spread across large areas and infect whole cities and towns.

When there was no access to equipment such as sophisticated microscopes that could pick up miniscule organisms, knowledge of how the world worked was largely limited to everyday observations, and they seemed to support the miasma theory. After all, measures taken to eliminate bad smells, such as paying more attention to personal hygiene, cleaning your living spaces, proper waste management, and moving slaughterhouses further away from densely populated areas, also accidentally got rid of germs, and therefore targeting pungent smells genuinely seemed to help. Marshy areas were also known to be hotbeds of infectious diseases such as malaria and yellow fever, and since we were not yet aware that these diseases can be spread by mosquitos, this seemed to confirm the theory that disease was generated by decomposing organic matter.

Other measures taken against epidemics based on the miasma theory were more misguided. Physicians, such as Thomas Lodge (c.1558-1625), recommended that bonfires should be burnt on street corners at least twice a week because the fire would consume the evil vapours and purify the air. The smoke rising from the bonfires, of course, only worsened the air quality in large urban centres and potentially increased the risk of respiratory infections. In addition, people attempted to ward off disease by replacing bad smells with more pleasant ones. The same Thomas Lodge, in his 1603 Treatise of the Plague, recommends using perfumes and essential oils to keep your home plague-free:

[…] it is necessary to make good fumes in our houses, of sweet and wholesome wood, [such] as Rosemary, Juniper, Laurel or Bay, and to perfume the whole house and chambers with […] Incense, dried Roses, Lavender, and such like, both Evening and Morning.

People also wore scents while out and about to protect themselves, and many medical books included recipes for perfumes that were thought to be effective in fighting various diseases.

Although some actions taken based on the miasma theory were more or less successful, the repeated epidemics that devasted Europe for centuries proved that many of the remedies were inadequate, and the theory also failed to provide an explanation for diseases that seemed to clearly originate from unsanitary drinking water. This made some medical professionals sceptical of the miasma theory and caused them to investigate alternative theories, including the germ theory of disease which finally became widely accepted in the medical community by the end of the 19th century.