The hygiene hypothesis is the idea that humans have become too concerned with germs and that the use of items like hand sanitizers and over cleaning has caused a shift in our immune systems. To some, this shift is to blame for the rise in allergies and auto immune diseases. When there are no germs for our immune systems to fight, they turn to benign things like food.
This idea is becoming more popular, and increasingly credible in the medical community.
A recent article from Australian Herald Sun explains the issue in more detail.