Complex sentence: The germ theory was not established at this point (Louis Pasteur would not propose it until 1861), so Snow was unaware of the mechanism by which the disease was transmitted, but evidence led him to believe that it was not due to breathing foul air.
Split into simpler sentences: The germ theory was not established at this point (Louis Pasteur would not propose it until 1861). Snow did not understand the mechanism by which disease was transmitted, but the evidence led him to believe that it was not due to breathing foul air.