I am guessing this question is motivated at least in part by the emergence of Covid-19. There is plenty of evidence in the geological record that bacteria existed a very long time before humans made their appearance on the earth. In fact, it is considered nearly certain that certain kinds of photosynthetic bacteria put oxygen into the atmosphere in the very distant past, changing the environment so that more complex life, such as animals could survive. Bacteria are absolutely essential to the maintanence of human and other life. Surely Adam and Eve had bacteria in their gut, otherwise they would not have had vitamin K and therefore their blood would not have clotted. I believe that God has not left evolution to chance and that he has intervened at various times and in various ways that I have no way of determining, but the facts prove beyond a reasonable doubt that bacteria and various parasites predate humans, and therefore predates the fall of mankind. There is also indirect evidence for viruses in the distant past as well. This is not fossil evidence or from the evolution of the atmosphere, but there are what are known as “viral insertions” into genetic material of various species from a time before there were any humans. I suggest you consider reading one of the books “The Language of God” by Francis Collins or “Coming to Peace With Science” by Darrel Falk. You will find them helpful.