The original meaning of “miasma” referred to actual clouds of toxic fumes — its Greek root means “pollute.” From the times of ancient Greece to as late as the mid-19th century, the miasma theory was an accepted medical belief that some illnesses were caused by a vaporous, disease-bearing cloud. Eventually, germs were found to be the true culprits of disease and infection, and now the word “miasma” is more commonly used in figurative contexts.