Answer : The correct answer is -
microorganisms.
Germ theory was first proposed by a French scientist, Louis Pasteur. According to this theory, microorganisms that are called as pathogens (disease causing agents) or "germs" can result into disease. In other words, growth and reproduction of microscopic organisms (microorganisms) within their respective host can lead to production of disease.
Example- Disease Cholera is caused by the bacterium Vibrio cholerae.
Thus, microorganisms is the right answer.