Miasma was caused, the theory went, by poisonous fumes emanating from both venomous animals (snakes, etc.) and decomposing bodies. These elements, sometimes called ‘effluviums’, were the actual things responsible for diseases. The air itself was literally the cause of disease.
In this way, the foul smell of air around marshes and swamps, and on some rivers (such as the Thames in London) was seen as indicative of its nature as the source of disease.
It’s easy to see this applied to a fantasy RPG. Those entering certain areas, particularly those adjacent to areas of stagnant, foetid water, or those frequented by poisonous snakes and other creatures, would be more likely to be struck by a disease of some sort. This is actually already reflected in the AD&D rules, which give modifiers to those in crowded cities and in marshes, making them more likely to contract some sort of disease. Too, the areas around certain temples dedicated to the princes of daemonkind would be similar hotspots of disease. Such places could be combatted, not with disinfectant, but with strong winds to blow away the disease-causing miasma…
Ironically, the misasma theory of disease did lead to some sanitation-related health reforms, due in part to false connections between improved sanitation and the remittance of disease. But it wasn’t until germs and viruses were really discovered to be the cause of such diseases that surface disinfecting became the norm, as we see today.