The true statement is: There was a consensus among scientists that miasma caused diseases before the 19th century.
During much of the 18th century and before, the miasma theory, which posited that diseases were caused by "bad air" or noxious vapors, was widely accepted. By the late 19th century, with the advent of the germ theory of disease, the scientific consensus shifted towards the understanding that germs (microorganisms) were responsible for many diseases.