Sanitation more than likely played some role in pre-vaccine times but another factor was living arrangements.
People were moving out of the big cities and the population boomed in the Midwest and middle America.
Large cities were a Petri dish of diseases and spread.
The final nail in the virus coffin was vaccines.
Like i sa8d, i think,people greatly underestimate exactly what clean water and effective sanitary sewer treatment processes have done to eradicate so many communicable diseases.
Moving to the suburbs and spreading out wouldnt have been successful if the water/sewer technologies hadn’t come along with them. They got the newer systems and new water and sewer procesing texhnologies yas they spread out.