Doctors once believed that illness was caused by foul air and could not be transmitted from one person to another. now, doctors know that single-celled bacteria and even smaller viruses cause illnesses and can be transmitted from one person to another. how did the advent of the microscope most likely affect the foul-air theory? it supported the theory of foul air causing illness and the theory become a law. it provided evidence for other causes of illness, but scientists did not change the theory. it allowed scientists to change the theory before any new information was collected. it allowed scientists to see small organisms in sick individuals and change the theory.