Pasteur observed microscopically tartaric acid crystals formed in wine. He observed two types of crystals that had mirror-image structures. These mirror-image structures have different molecular structures that can influence their properties.

While experimenting with vats, organisms would often grow on his yeast extract medium. He deduced that these organisms were being introduced from the air.
Take a piece of meat from a dead animal, let it stand and soon it's teeming with life. Spontaneous generation was the hypothesis that living organisms can originate from non-living matter. Pasteur disproved this idea through a famous experiment: the swan-neck experiment.

• Causative agents of several human diseases do not cause disease in any (model) animal
• Many bacteria do not grow in pure cultures on plates
• Viruses (later discovered) cannot be grown in pure culture on plates