"The 'hygiene hypothesis' which became widely publicised in the 1990s argued that rising rates of allergies were being caused by 'overcleanliness'..."

"... suggesting children should be exposed to a wide range of potentially harmful microbes. But experts from the Royal Society today stressed this is not the case. They said people need diverse exposure to microbes that are mostly harmless - such as those children can find playing outdoors - but should remain vigilent [sic] about hygiene in the home.... A survey of 2,000 adults found that almost one in four people agreed with the statement 'hygiene in the home is not important because children need to be exposed to harmful germs to build their immune system'. More than half also mistakenly thought keeping homes too clean was damaging.... The survey also revealed 'substantial public confusion' about the relationship between cleanliness and hygiene, with 61 per cent believing dirty hands from outdoor play are likely to spread harmful germs, despite there being little evidence that outdoor dirt carries harmful microbes."

From Yahoo News.

I think everyone is clean about some things and dirty about others. The key seems to be to choose the right things to be clean about.