Objective: To investigate the working hypothesis that common infections occurring early in life prevent atopy.

Design: Cross sectional, retrospective study of young Italian men with results for hepatitis A serology and atopy.

Setting: Air force school for military students in Caserta, Italy.

Subjects: 1659 male students aged 17-24, most of whom (90%) were from central and southern Italy.

Main outcome measures: Skin sensitisation and specific IgE antibodies to locally relevant airborne allergens; diagnosis of respiratory allergy (asthma or rhinitis, or both); hepatitis A seropositivity.

Results: 443 of the 1659 subjects (26.7%) were positive for hepatitis A virus antibody. Atopy was less common among seropositive than seronegative subjects according to skin sensitization (weal reaction ≥3 mm) to one or more allergens (21.9% (97/443) v 30.2% (367/1216), P<0.001); polysensitisation (sensitive to three or more allergens) (2.7% (12/443) v 6.4% (78/1216), P<0.01); high specific IgE concentration (9.7% (43/443) v 18.4% (224/1216), P<0.00005); and lifetime prevalence of allergic rhinitis or asthma, or both (8.4% (37/443) v 16.7% (203/1216), P<0.001). Hepatitis A seropositivity remained inversely associated with atopy after adjusting for father's education, the number of older siblings, and the area of residence (based on the number of inhabitants). The prevalence of atopy was constantly low among seropositive subjects, whatever the number of older siblings; by contrast, it increased with a decreasing number of older siblings among seronegative subjects.

Conclusion: Indirect but important evidence is added to the working hypothesis as common infections acquired early in life because of the presence of many older siblings (among seronegative subjects) or because of unhygienic living conditions (among seropositive subjects) may have reduced the risk of developing atopy.

• Young men with antibodies to hepatitis A virus had a lower prevalence of atopy and atopic respiratory diseases, and this was independent of the number of older siblings and other relevant risk factors
• The prevalence of atopy was as low in seronegative as in seropositive subjects only when they had three or more older siblings
• Among seropositive subjects the prevalence of atopy was low, whatever the number of older siblings
• Common infections acquired early in life because of the presence of many older siblings (among seronegative subjects) or because of unhygienic living conditions (among seropositive subjects) may have reduced the risk of development of atopy
• This study adds indirect but important evidence to the hypothesis that improvements in hygiene and reduced recirculation of common infections may be a major cause of the increasing prevalence of atopy and atopic diseases in Western countries