In the 19th century we learned that we needed clean drinking water to avoid waterborne diseases. This recognition eventually led to the development of national scale sewage disposal and treatment and the collection and purification of drinking water.
When it comes to viruses like Covid 19 and flu, we know from the increased transmission rates we see in the Winter that indoor air is a similar problem.
Much work has been done to demonstrate that airborne transmission via aerosol is the major transmission route eg: