Miasmatic Theory

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Back in the Medieval and Renaissance periods, there was no understanding of microbes as a cause of disease. Generally speaking, disease states were understood as products of internal imbalance within the individual. If you had a cold, for example, it was thought that you had too much cold and damp in your body, which needed to be expressed as phlegm - one of the four humours (the other three were black bile, yellow bile, and blood).

The Four Temperaments / Humours of Galen
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Generally speaking, this is a highly functional medical model, despite the idiosyncracies of some of its terminology. If your body is healthy and in balance, you will be better able to fight most microbes, after all.

When it came to bubonic plague, however - a disease that could fell a perfectly healthy person in a matter of days, and that caused large quantities of black pus / blood to pool in lumps at the surface of the skin - medieval and Renaissance medical people knew they needed something more to explain what was going on.

That's where miasmatic theory comes in.

Although the idea of microbes was still a little way off, early doctors proposed the idea that plague was caused by infection from outside the individual. The cause? Foul air, or "miasma." The understanding of what made air foul was pretty broad. A "miasma" could mean bad, stinking breath, rotten smells, or even the "atmosphere" in places where people were crowded together and overexcited - like the theatre. (But not church.)

This obviously posed a huge challenge to London city dwellers throughout the Renaissance. London was a notoriously stinky place. Open sewers, mass graves, and slaughterhouses within the city walls all proved a challenge when it came to avoiding stink.

Practices arose designed to counteract bad smells during plague time. The famous plague doctor's mask, resembling a bird beak, contained pleasant-smelling herbs at the end. Inhaling through the herbs, the wearer could avoid exposure to bad smells and thus hopefully avoid infection.

Plague Doctor's Outfit
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In the sick room, doctors distributed herbs and flowers, or heated them on warm bricks in order to dispel the infected air.

A doctor attempts to freshen the air in a plague house.
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The practice of carrying a nose-gay - a small bouquet of flowers - arose during the same time. Any time you encountered a bad smell, you could sniff the flowers, and thus decontaminate yourself. Sachets - nice-smelling small pillows stuffed with herbs and flowers - are a worldwide practice. In the east, they are thought to drive away evil spirits. In the west, they originated as a remedy for disease.