I am not so sure but this is what I think.
People initially thought that microorganisms spontaneously appeared on human beings and maggots spontaneously appeared in the meat (read about THEORY OF SPONTANEOUS GENERATION). When it was proved that actually maggots don't appear in meat when flies were prevented from laying eggs by Francesco Redi, then this paved way for theory of biogenesis. 17th century scientists later on believed that spontaneous generation was false. Thus, the established theory was "omne vivum ex ovo" which means that every living this came from a pre-existing living thing/egg.
Theory of biogenesis meant that microbes were giving rise to other microbes and microbes were not a result of a disease but the reason. Thus, this paved the way for germ theory of life.
(I am sorry - I don't have much time so my answer is not very articulate but I hope you understand).