What the Germ Theory suggests is that "certain diseases are caused by the invasion of the body by microorganisms, organisms too small to be seen except through a microscope." We now define this not as merely a theory but as common sense. But at the time when the French chemist and microbiologist Louis Pasteur, the English surgeon Joseph Lister, and the German physician Robert Koch came up with the theory, this was not the case. Humans still believed that every bad smell necessarily held threatening health risks.
Such was the case in the months of August and September of 1880 in Paris. The city was taken hostage by a stench, and the horrible smell caused a scare in the city, as it was taken as a signifier of sickness. The origins of the bad smell were the sewers. The city was flooded with filth. As the journal Le Figaro put it, "It stank to high heaven."
The scare quickly birthed rumors of sickness. These rumors spread so quickly and to such a great extent that citizens began to protest, and a government commission was set up to deal with the issue, announcing that the foul smell could harm public health. As time passed, the stench went away with it, and the people of Paris saw that they were, in fact, left unharmed, so they started to embrace the Germ Theory. When 15 years later, the city was once again suffocated by the stench, the media coverage was almost humorous about it - it wasn't a cause for a health panic anymore. It was nothing more than a mere inconvenience.
Pasteur, Lister, and Koch worked hard between 1850 and 1920, conducting various experiments with clear conclusions to persuade the public to believe in the Germ Theory and adopt good hygiene habits. Still, it took many more years until the public and medical practitioners adopted good sanitation habits.
Operating surgeons weren't disposing of hazardous waste properly, and they wore unsanitized and sometimes stained garments during surgeries. Both in the workforce and at home, hand washing was not a common custom. Sewage systems weren't separating waste from drinking water properly. But all of this wasn't because people weren't concerned with hygiene. They were simply not aware that these actions posed health risks. This is hard to imagine in an age where every toddler knows to wash their hands before their meal.