The obvious answer is the sanitary revolution of past 200 years.
Improved water quality and sanitation:
- occurred gradually from 1800s, but widespread coverage (toilets, treated water, etc) did not occur until late 19th into C20th
- environmental clean-up from 1850s - reduced exposure to human waste and animal excreta in city streets.
- Rapid decline in mortality 1900 - 1950 particularly young children.
- Decline in cholera, typhoid, TB, diphtheria, and common childhood infections
The link to the sanitary revolution may be correct, but, if the OF mechanism is also correct, the likely explanation is that these changes have also inadvertently reduced exposure to our Old Friends which occupy the same habitats.