The incidence of auto-immune diseases like type 1 diabetes and allergies has risen dramatically in developed countries over the past fifty years .

Are allergies and so-called autoimmune diseases, like type 1 diabetes, likely to be caused by a lack of exposure to relatively benign pathogens in early childhood? This theory is referred to as the hygiene hypothesis.

Bacteria and other microorganisms that cause infections are remarkably resilient and can develop ways to survive drugs meant to kill or weaken them.