title: The hygiene hypothesis: does it function worldwide?
abstract: OBJECTIVE
  This article intends to be a systematic review of papers published during 2003-2004 quoted in a Medline search for 'hygiene hypothesis'. The worldwide perspective of the article does not refer just to a geographical concept, but tries also to address the question of whether the consensus on the hypothesis is global or whether it applies to all types of allergic or immunologic disease, to any age sample or infectious agent. Furthermore, the article outlines those clinical and experimental studies which, in the authors' opinion, may represent significant contributions to a better understanding of the hygiene hypothesis and may guide further investigations on the subject.
  
  
  RESULTS
  The association between a reduced exposure to infectious agents (as a part of a changed lifestyle) and a higher prevalence of atopy seems now to be confirmed by consistent evidence. Mechanisms underlying this association, however, are not yet completely clear (immune deviation or immune regulation).
  
  
  CONCLUSIONS
  Further experimental and clinical studies are needed, with special reference to the time, duration and intensity of exposure to any specific infectious agent which is related to well-defined allergy outcomes. The background information for using microbial products in allergy prevention and treatment is still limited.
authors.affiliation: IRCCS San Raffaele Pisana, Research Center, Rome, Italy. megon.bresciani@sanraffaele.it
authors.forename: Megon
authors.initials: M
authors.lastname: Bresciani
authors.forename: Claudio
authors.initials: C
authors.lastname: Parisi
authors.forename: Ginevra
authors.initials: G
authors.lastname: Menghi
authors.forename: Ginevra
authors.initials: G
authors.lastname: Manghi
authors.forename: Sergio
authors.initials: S
authors.lastname: Bonini
mesh_terms: D004781:Environmental Exposure; D006801:Humans; D006920:Hygiene; D006967:Hypersensitivity / Q000209:etiology* / Q000276:immunology / Q000382:microbiology