List and describe as many changes in human behaviors as you can think of that contribute to decreased exposure to microbes:
Increased handwashing and with antibiotic soap [1]
Increased cleaning/cleaning products
Kids not playing outside
Kids not playing with other kids
Increased antibiotics [1]
Hand sanitizer
Smaller families (less children) [1]
More C-sections [1]
Less breast feeding [1]
More antibiotics during pregnancy [1]
It was also mentioned that we may have a loss of ancestral microbes (1)
Blaser M, Falkow S. 2009. What are the consequences of the disappearing human microbiota? Nature Reviews Microbiology 7:887–894.