Hygiene Hypothesis

The hygiene hypothesis proposes that reduced early-childhood exposure to microorganisms, parasites and common infections, a result of smaller family sizes and improved sanitation and hygiene in modern societies, disrupts the normal development of the immune system and contributes to rising rates of allergic and some autoimmune conditions. Epidemiologist David Strachan introduced the idea in a 1989 paper in the British Medical Journal, after observing that children in larger families with more older siblings had consistently lower rates of hay fever and eczema than children with fewer siblings. The hypothesis has been refined considerably since its original form, with later researchers proposing that specific, largely harmless microbial exposures, rather than infectious illness itself, are what properly calibrate immune development, an idea sometimes called the old friends hypothesis. It remains an active and still-developing area of immunological and epidemiological research rather than a fully settled explanation.