Mass Use of Antibiotics
By the time a child reaches age 18 in the industrialized world, the chances are he or she has been given 10-20 courses of antibiotics. That misuse continues into adulthood, and they're casually prescribed to pregnant women.
That's where the situation grows ever worse. Part of normal childbirth is a baby's passage through the birth canal—where it's exposed to its first dose of beneficial bacteria. (This should give pause to anyone considering a caesarian birth that isn't absolutely necessary.)
When a mother's microbiota is deficient, her child is born to that deficiency. The evidence now appears to show that, once a probiotic deficiency exists, it is never recovered—and it's passed down the generations. Therefore, each generation is likely to suffer from poorer health than the parents enjoyed.