Background - The ‘hygiene hypothesis’ has been used to justify a belief common among homeopaths, that the suppression of childhood infections and immunisation may lead to the development of chronic atopic diseases.Objectives - To analyse the influence of childhood infections and immunisation on the development of atopy.Methods - Qualitative systematic review of direct epidemiological evidence (Medline 1993–2004) concerning the influence of childhood infections and immunisation on the development of atopy and discussion based on homeopathy.Conclusions - (1) Childhood infections do not protect against atopy; on the contrary, they increase the risk of allergic diseases, in agreement to Hahnemann's observations, which included epidemic diseases among the factors capable of stimulating the development of chronic diseases. (2) Vaccination is not a risk factor for atopy, notwithstanding the allergenic effect of some vaccines.