For more insight about historical background of Spontaneous Generation see Abiogenesis.
Spontaneous generation is the idea that living organisms can be spontaneously formed from non-living substances. Other definitions refer to it as to the concept that animal forms are created by the decay of organic matter . Common theories included:
- Moldy grain produces mice
- Rotten meat produces maggots
- Rotting garbage produces rats
Experiments in the 1600s through the 1800s discredited this idea. These experiments took the form of allowing meat to rot in sealed and semi-sealed containers (Francesco Redi, 1668), which demonstrated that maggots only formed in meat that flies laid eggs in; and by storing sterilized soup in bottles that prevented dust and bacteria from entering (Louis Pasteur, 1860).
It was still the scientific consensus when Charles Darwin wrote Origin of the Species in 1859 that microbes arose by spontaneous generation. 
- Wile, Dr. Jay L. Exploring Creation With General Science. Anderson: Apologia Educational Ministries, Inc. 2000