Clinical and subclinical mastitis are common among lactating women but their potential influence on programmes to improve infant health through maternal vaccination has not previously been addressed. Maternal antibodies, acquired either through natural infection or through vaccination, enter milk through two main pathways: by transcytosis or paracellularly. Transfer through both pathways is likely to be increased during mastitis, mediated by inflammatory cytokines. The increases are of similar order of magnitude to the variation in milk antibody concentrations between women within a population or between women in developing and developed countries. Thus, the effects of mastitis on vaccination programmes can be subsumed under the larger question of how breastfeeding in general affects these.