A review of studies on hygiene and the microbiota published in the Journal of Allergy and Clinical Immunology has shown that the myth of the 'too clean home' not helping to build up the immune system is a false one. This is because it has actually been found that excessive household hygiene has no major effect on the ability to repel the attack of viruses and bacteria. Not only that, it has also been shown that children build up a healthy immune system even in a clean home.
According to the hygiene hypothesis (which advocates moderate exposure to germs and bacteria), letting children be exposed to microorganisms that have evolved alongside humans helps to develop an efficient immune system, and to ward off allergies and autoimmune diseases.
However, the transfer of 'good' micro-organisms to children occurs mainly through social interactions, first with the mother and then with close family members. The real factors that reduce this transfer and increase the likelihood of problems with immunological regulation are a caesarean section, lack of breastfeeding and other opportunities for closeness between mother and child, as well as frequent use of antibiotics and inadequate nutrition.