However, a lower incidence of gastrointestinal infections in breastfed infants in the United States (Myers et al., 1984) and lower perinatal mortality from infectious disease among breastfed, compared with formula-fed, infants in England (Carpenter et al., 1983) suggest that the protective effects of breastfeeding are important in industrialized as well as in developing countries. Moreover, since breastfed infants may remain asymptomatic even after exposure to enteropathogens (microorganisms that infect the intestinal tract) contaminating the nipples and areola of the nursing mother (Mata and Urrutia, 1971; Wyatt and Mata, 1969), the protection afforded by breastfeeding is attributable to factors in the milk in addition to an avoidance of pathogens.
For some infections, the duration is shorter and the intensity is less in breastfed infants than in formula-fed infants, but the attack rate (the incidence during a defined period of risk) is similar (Duffy et al., 1986; Glass et al., 1983; Mata et al., 1967, 1969). For example, in Bangladesh, protection against infections but not against colonization with Vibrio cholerae correlated with the amount of secretory immunoglobulin A (IgA) antibodies to that bacterium in the human milk (Glass and Stoll, 1989; Glass et al., 1983). In the United States, the protection afforded to infants against rotavirus appeared to result from factors in human milk other than specific antibodies (Duffy et al., 1986).
Protective effects of breastfeeding against certain respiratory infections have also been reported. For example, respiratory syncytial virus infections are less severe among breastfed infants than among formula-fed babies, although the attack rate is the same (Chiba et al., 1987; Downham et al., 1976; Pullan et al., 1980). In addition, serum interferon-α levels are higher in breastfed infants following respiratory syncytial virus infection (Chiba et al., 1987), even though the concentration of this antiviral agent is negligible in human milk. This suggests that the breastfed infant's immune system may be primed by human milk to respond to certain respiratory viral pathogens more rapidly and to a greater degree than is the case for formula-fed infants.
The incidence of otitis media is lower in breastfed than in formula-fed infants (Saarinen, 1982; Schaefer, 1971), but it is unclear whether this is a result of components of human milk or of other factors associated with the feeding method, such as higher enrollment of formula-fed infants in day-care centers, differences in the sucking mechanisms, or the adverse effects of foreign proteins in formula-fed infants.
The relationship between the nutritional status of the lactating woman and the susceptibility of the breastfed infant to infections requires further study.
The effects of various infant-feeding practices on the age of onset and severity of allergies have been extensively investigated. Breastfeeding appears to be protective against food allergies (see the review by Kramer ).