Keratinizing squamous metaplasia of lactiferous ducts may play a similar role in the pathogenesis of nonpuerperal subareolar abscess. Causes
Since the 1980s mastitis has often been divided into non-infectious and infectious sub-groups. However, recent research  suggests that it may not be feasible to make divisions in this way. It has been shown that types and amounts of potentially pathogenic bacteria in breast milk are not correlated to the severity of symptoms. Moreover, although only 15% of women with mastitis in Kvist et al.s study were given antibiotics, all recovered and few had recurring symptoms. Many healthy breastfeeding women wishing to donate breast milk have potentially pathogenic bacteria in their milk but have no symptoms of mastitis. Risk factors
Mastitis typically develops when the milk is not properly removed from the breast. Milk stasis can lead to the milk ducts in the breasts becoming blocked, as the breast milk is not being properly and regularly expressed. It has also been suggested that blocked milk ducts can occur as a result of pressure on the breast, such as tight-fitting clothing or an over-restrictive bra, although there is sparse evidence for this supposition. Mastitis may  occur when the baby is not appropriately attached to the breast while feeding, when the baby has infrequent feeds or has problems suckling the milk out of the breast. The presence of cracks or sores on the nipples increases the likelihood of infection. Tight clothing or ill-fitting bras may also cause problems as they compress the breasts.
It is of tremendous economic importance for the dairy industry, and it is also of concern for public health. The same considerations apply to mastitis in sheep and goats and other milk-producing females. It is also of economic importance in the sow, but, in this species, it is not related to public health. In other domestic females (queen, mare,  etc. ), it is more an individual illness dealt with by veterinary practitioners. References
External links

Mastitis on mayoclinic.com
Australian Breastfeeding Association