For millennia, disease was bad smells and imbalanced humours. Then Pasteur killed spontaneous generation with a swan-neck flask, and Koch proved microbes cause disease with four simple rules. Let time pass and watch what reaches the broth.
Exposing broth to air — is the flask shape protecting against microbes or just "bad air"?
Pasteur's swan-neck flask let air in but trapped dust and microbes in the curve. Broth stayed clear for months. Break the neck, and within days the broth teemed with life. Spontaneous generation was dead — microbes came from microbes, not thin air. This simple experiment forced medicine to look for living causes of disease, not bad vapours or moral failings. Pasteurisation applied the same insight: heat kills microbes, careful handling keeps them out.
Germ theory replaced miasma once Pasteur's swan-neck flasks disproved spontaneous generation and Koch's four postulates proved that specific microbes cause specific diseases, unlocking antisepsis, vaccines, and modern public health.