Invisible and usually foul-smelling vapors thought to be harmful. At this time, medical science did not fully understand the role of bacteria and viruses in causing disease. Doctors and scientists thought many diseases were spread by effluvia, emanations, or miasmas that arose from filth and decaying substances.

Exhalations or emanations, applied especially to those of noxious character. In the mid?nineteenth century, they were called "vapours" and distinguished into the contagious effluvia, such as rubeolar (measles); marsh effluvia, such as miasmata; and those arising from animals or vegetables, such as odors.