Complex sentence: Even before Pasteur's work on the nature of infection there was evidence that contagions spread from person to person, even though microscopes were not yet available and the nature of the contagion (microorganisms) could not be elucidated.
Split into simpler sentences: Even before Pasteur's work, there was evidence that contagions spread from person to person. Before the availability of microscopes, the nature of that contagion (microorganisms) could not be elucidated.