Recent decades have seen a significant shift in the main causes of illness among infants and young children. Until the 1950s and 196Os, acute infectious diseases affecting the gastrointestinal tract were the most common conditions requiring medical attention. This pattern of morbidity persists to this day in the poorest regions of the world and still claims the lives of millions of children. In developed countries, however, and also in developing countries where successful programmes of vaccination and prevention have been established, chronic, recurrent illnesses of the respiratory tract have taken the place of acute diarrhoea. A very high proportion of children (up to half in populations of northern European origin) are affected by allergies, and the prevalence of asthma and other forms of recurrent airway obstruction is rising.’Paradoxically, the increases seem most striking in areas where the quality of outdoor air has significantly improved’and where good sanitation and nutrition prevail.

Some years ago we separately proposed3,* similar hypotheses to explain these findings. This so-called “hygiene hypothesis” was an immunological extension of a concept introduced by Strachan. 5 Its cornerstone is that changes in the type and level of stimulation from the microbial environment associated with improvements in public health and hygiene may have indirectly influenced the postnatal development of immune function (s), so as to increase predisposition to chronic allergic conditions during childhood. These changes involve both a general decrease in infectious burden during early life (in particular, respiratory infections) and variations in gastrointestinal flora. In this paper, we review how recent advances in understanding of the way immune responses develop in early life support the hygiene hypothesis.