I believe that disease rates plummeted with the introduction of the automobile and generally faster means of transportation. You see, as people began to move faster, the various diseases couldn’t keep up with them. The diseases were too slow to infect people. Especially with airplanes and faster trains, disease rates dropped further.
Now, of course you’ll say ‘but people who have travelled in these faster means of transportation have also contracted (insert name of disease here)’. That’s because some individual strains of the disease mutated to be able to move faster. But by and large, people in developed countries are moving at a speed too fast for most versions of the disease to catch.
In third world countries, where there’s much less developed means of travel, disease rates are much higher.