Infections presenting for clinical care in early life and later risk of hay fever in two UK birth cohorts.

The ‘hygiene hypothesis’ proposes that infections in infancy protect against hay fever (HF). We investigated infections during infancy in relation to HF, including rarer ones not previously researched in this context, while examining the role of potential confounding variables. Our results indicated that upper respiratory tract infections, diarrhoea and vomiting and acute otitis media in infancy were each related with a moderately increased risk of HF in both databases, as were BP. Of 30 infectious illnesses investigated, none had strong or consistent associations with HF after adjustment for consultation frequency. Except for bronchiolitis, possibly a chance finding, none of the clinically apparent infections considered appear to have an important role in allergy prevention. Allergy 2008:63:274–83.