Only around 1860, with experiments carried out by Louis Pasteur (1822 - 1895), it was definitively proved that microorganisms arise from other pre-existing ones.
Pasteur's experiments are described and outlined in the figure below:
The absence of microorganisms in the “swan neck” vials kept intact and their presence in the vials whose “neck” had been broken show that the air contains microorganisms and when they come into contact with the nourishing and sterile balloon fluid. , they develop. In the intact flask, these microorganisms cannot reach the nourishing and sterile liquid because they are trapped in the “filter” formed by water droplets that appear on the balloon's neck during cooling. In vials in which the neck is broken, this “filter” ceases to exist, and airborne microbes may come into contact with the nourishing liquid, where they find suitable conditions for their development and proliferate.
Since then, the hypothesis of biogenesis has been universally accepted by scientists.