If you have a cracked nipple and it does not heal, you will probably end up with mastitis - a breast infection. Bacteria from your baby mouth enters your breast through the crack or through the nipple openings. Breast milk is very nutritious so the bacteria will multiply and cause you pain. Photo shows scanning electron microscopy of Staphylococcus aureus, a common bacteria which causes mastitis.
Your breast will be sensitive to touch, it may have lumps & bumps which is where the bacteria is multiplying and clogging your milk ducts. You will have flu like symptoms: fever, aches & pains and the general feeling of being unwell.
Breast infections can go away by themselves. Your body has to be strong enough to fight off the infection. To help your body fight off the infection, empty out the infected breast of milk at every feeding. By flushing out the milk in your breast, you will flush out the bacteria. What ever bacteria that remains in your breast will be deprived of food and will multiply less readily. Your immune system then destroys the remaining bacteria.
One side affect of antibiotics is that it will also kill naturally-occurring, beneficial bacteria in your body. When naturally occurring, good bacteria are eliminated, it leaves a void where other detrimental organisms may live. In other words, antibiotics may cure you of a breast infection, but in doing so, it may cause secondary infections such as yeast infection.
Antibiotics can be transferred to your baby through breast milk, so your babys bacterial flora will be affected too. Thus, you must be especially careful: have a healthy diet and a healthy lifestyle so you and Baby don't get secondary infections.