From Williams Obstetrics, 19th Edition, p. 647

Mastitis - The most common offending organism is Staphylococcus aureus; Matheson and colleagues (1988) culture this organism from 40 percent of women with mastitis. . . . The immediate source of the organisms that cause mastitis is almost always the nursing infant's nose and throat. At the time of nursing, the organism enters the breast through the nipple at the site of a fissure or abrasion, which may be quite small. . . .

Mastitis among nursing mothers has at times reached epidemic levels. Such outbreaks most often coincide with the appearance of a new strain of antibiotic-resistant Staphylococcus, an example being methicillin-resistant S. Aureus (MRSA). Typically, the infant becomes infected after contact with nursery personnel who are colonized. Attendants' hands are the major source of contamination of the newborn. Especially in a crowded, understaffed nursery, it is a simple matter for the personnel to inadvertently transfer staphylococci from one colonized newborn infant to another. The colonization of staphylococci in the infant may be totally asymptomatic or may locally involve the umbilicus or the skin; but occasionally the organisms may cause a life-threatening systemic infection.

Once established, resistant staphylococcal infections tend to spread and recur among the family for protracted periods of time.
From Williams Obstetrics, 19th Edition, p. 1281

A major mechanism for inducing infection subsequent to birth is from those caring for the infant who may be colonized with the organism or may passively transfer it from another infected infant.