Breast milk contains antibodies, so breast fed infants may have slightly longer protection from viruses. The first cold will usually occur around three or four months of age if the infant is exposed to other infants that have colds. If infants stay at home for the first five years of life and have no exposure, they will not get any colds. This can actually be undesirable because the child will be exposed to numerous viruses when they start going to school. They will develop the same pattern of infection as infants who will usually get between 10 and 15 colds per year in their first two years of exposure.
Children can miss so much school, that they end up having to repeat the first year.
Certainly it is better to start exposure after the infant is several years old, but in today's society, frequently, both the mother and father are working and the infant has to go to a day care center. The frequency of colds per year is directly proportional to the number of children in the day care center. If there are over 10 children, the infant will get about 15 colds per year. If there are only about two or three children, then the infant may only get 10 colds per year. The infant is not susceptible to another virus for 2 weeks after the onset of one virus, because of the production of interferon.