The true statement is: "There was a consensus among scientists that germs caused diseases by the 19th century."
The germ theory of disease gained prominence in the 19th century, particularly through the work of scientists like Louis Pasteur and Robert Koch, leading to a broader acceptance of the idea that microorganisms were the cause of many diseases. Prior to that, many different theories, including miasma or "bad air," were prevalent, and there wasn't a consensus specifically about germs causing diseases.