Who developed the germ theory of disease?

The germ theory of disease was developed primarily by Louis Pasteur and Robert Koch, making this the correct answer. Louis Pasteur's experiments established that microorganisms were responsible for fermentation and spoilage, leading to the understanding that these microbes could also be responsible for diseases. His work with pasteurization and vaccines further solidified the idea that specific germs could cause specific diseases.

Robert Koch built upon and supported Pasteur’s work through his methodical research identifying the causative agents of diseases, such as anthrax and tuberculosis. He formulated Koch's postulates, a series of criteria to establish a causal relationship between a microbe and a disease, fundamentally shaping microbiology and infectious disease study.

The other options feature prominent figures in microbiology and medicine, but their contributions do not encapsulate the establishment of germ theory in the same direct way. Alexander Fleming is known for his discovery of penicillin, a pivotal discovery in antibiotics rather than germ theory. Joseph Lister is famous for developing antiseptic surgical methods that reduced infections but did not contribute directly to germ theory. Edward Jenner is known for developing the smallpox vaccine, which was an important advancement in immunology, while Paul Ehrlich made significant strides in chemotherapy, but neither directly worked on the foundations of germ theory