"Cleanup" redirects here. For the Wikipedia term, see Cleanup.

Cleanliness is the absence of dirt, including dust, stains and bad smells. In more recent times, since the germ theory of disease, it has also come to mean an absence of germs and other hazardous materials. A recent shift has now taken place to recognise that ‘dirt’ may play a useful role in our immune systems. This shift in thinking can be traced back to 1989, when David Strachan put forth the "hygiene hypothesis" in the British Medicine Journal. Strachan looked at the records of 17,000 British children and found that the greater number of older siblings they had, the less likely they were to come down with hay fever—a disease which, despite its name, is far more common in the city than the country. Strachan wondered if the older children were bringing home more viral infections to their younger siblings, priming their immune systems so they could better tolerate pollen. The "hygiene hypothesis" has now been linked with asthma, allergies, intestinal diseases including Crohns disease, childhood leukaemia and atopic dermatitis and the list is growing. It can apply to humans, animals, clothing, eating utensils, plates, pans, cups, etc., food, other movable objects, floors, windows, walls, toilets, etc.

"Cleanliness is next to Godliness," a common phrase, describes humanity's high opinion of being clean.