The importance of breast-feeding to infant health is still debated in the United States, where the environment is predominantly one of clean water, good sanitation and readily available hygienic infant formula.
The researchers analyzed data on 7,092 babies from the National Maternal and Infant Health Survey (NMIHS), looking for any relationship between breast-feeding dose and illness in the first six months of life. Breast-feeding dose (ratio of breast-feedings to other feedings) was categorized as full, most, equal, less or none.
The results of this study suggest that full breast-feeding has a significant protective effect against common infant illnesses. Infants who had full breast-feeding had lower ratios of "sick baby medical visits and months of illness with diarrhea, cough or wheeze, ear infection, runny nose or cold, fever, vomiting, or pneumonia." The protective effect did not vary among income groups, meaning that full breast-feeding probably accounted most for the protective effects described. Minimal breast-feeding didn't provide comparable protection.
Raisler J, Alexander C, O'Campo P. Breast-feeding and infant illness: A dose-response relationship? American Journal of Public Health, Jan. 1999; vol. 89, no. 1, pp25-30.