There are a bunch of different variables affecting diseases that need to be considered.
Before the turn of the 20th Century, medical hand sanitation with carbolic acid radically lowered the number of postpartum infections in both mothers and infants.
The Pure Food and Drug Act of 1906.
Damming of rivers dried up many swampy, mosquito breeding areas.
The popularization of outhouses and indoor plumbing in the 1920’s wiped out many parasitic infections in surface soil.
The use of air conditioning had a big impact.
WWII era public health hygiene education.
But some credit as well needs to go to antibiotics and vaccines.
“But some credit as well needs to go to antibiotics and vaccines.”
Less that 4% (at best), according to the paper.