Mastitis is inflammation of breast tissue that causes pain, swelling, and redness, often accompanied by fever and flu-like symptoms. It most commonly develops during breastfeeding, but it can also affect people who aren’t lactating. The condition exists on a spectrum, ranging from mild inflammation caused by backed-up milk to a bacterial infection that requires antibiotics.
How Mastitis Develops
Breast milk naturally contains a diverse community of bacteria that coexist in balance. Mastitis typically begins when milk flow through a duct slows or stops, a situation called milk stasis. This can happen from skipped feedings, a poor latch, tight clothing, or anything that prevents milk from draining efficiently. When milk sits in the duct, the surrounding tissue becomes inflamed and swollen, which narrows the duct further and makes the problem worse.
As stasis continues, the bacterial balance inside the breast shifts. Bacterial diversity drops, and certain species start to dominate. During acute mastitis, the bacterium Staphylococcus aureus tends to take over, while a related species called Staphylococcus epidermidis is more common in slower-developing cases. This imbalance, sometimes called mammary dysbiosis, triggers further inflammation as the overgrown bacteria produce compounds that irritate the surrounding tissue. Not every case of mastitis involves a true infection, though. The inflammatory response alone can produce significant symptoms, including fever and chills, even without bacteria actively invading the tissue.
What Mastitis Feels Like
Symptoms tend to come on suddenly. The hallmark sign is a painful, warm area on one breast (though both breasts can be affected) with skin redness that often appears in a wedge-shaped pattern fanning outward from the nipple. On darker skin tones, this redness can be harder to spot, so swelling, warmth, and tenderness may be more reliable indicators.
Beyond the breast itself, mastitis often feels like getting hit with the flu. Fever at or above 101°F (38.3°C), chills, body aches, and a general feeling of being unwell are common. These systemic symptoms can be intense enough that some people feel sick before they even notice the breast changes.
Inflammatory vs. Bacterial Mastitis
Current guidelines from the Academy of Breastfeeding Medicine describe mastitis as a spectrum rather than a single condition. Understanding where you fall on that spectrum matters because the treatment is different at each stage.
Inflammatory mastitis is the earlier phase. The breast becomes increasingly red, swollen, and painful, and you may develop fever, chills, or a rapid heart rate. The key point is that these symptoms can happen without an active bacterial infection. Your immune system is reacting to the stalled milk and tissue swelling, not necessarily to invading bacteria. At this stage, the condition often responds to conservative measures like improved milk drainage, rest, and anti-inflammatory pain relievers.
Bacterial mastitis is a progression beyond that. The redness worsens and may spread to different areas of the breast, the tissue feels hard or thickened, and symptoms don’t improve with basic care. If you have a fever and other systemic symptoms lasting longer than 24 hours, or if the breast isn’t responding to conservative treatment, bacterial mastitis is the likely explanation, and antibiotics become necessary.
Mastitis in Non-Breastfeeding People
Mastitis doesn’t only happen during breastfeeding. A form called periductal mastitis affects the ducts just behind the nipple in people who aren’t lactating. The exact cause isn’t fully understood, but smoking is the strongest known risk factor. In one study of 139 patients diagnosed with periductal mastitis, 89% were smokers, compared to 39% of age-matched controls.
The connection appears to be chemical. Breast tissue concentrates substances from cigarette smoke, and cotinine (a breakdown product of nicotine) reaches higher levels in the ducts behind the nipple than in the bloodstream. These compounds may directly damage the duct lining or reduce oxygen supply to the tissue, leading to tissue breakdown and subsequent infection. Over time, the ongoing inflammation can cause structural changes in the ducts that make recurrence more likely.
Treatment and What to Expect
For early or mild mastitis, the focus is on reducing inflammation and keeping milk moving. Ice packs applied to the affected area, over-the-counter anti-inflammatory pain relievers, and frequent breastfeeding or milk expression are the foundation. Aggressive massage of the breast was once standard advice but is now discouraged in updated guidelines, as it can worsen tissue swelling. Gentle techniques to encourage milk flow are preferred.
If symptoms are severe or persist beyond 24 hours, antibiotics targeting Staphylococcus aureus, the most common culprit, are the standard treatment. A typical course lasts 7 to 14 days. It’s safe to continue breastfeeding while taking these medications, and in fact, continuing to nurse helps clear the infection by draining the affected tissue.
Some research has explored probiotics as both a treatment and prevention strategy. Specific Lactobacillus strains have shown effectiveness in clinical trials at displacing the Staphylococcus species that cause mastitis, essentially helping to restore the breast’s normal microbial balance. This approach is still more common in some countries than others, but it represents a growing area of clinical practice.
When Mastitis Gets Worse
The main complication to watch for is a breast abscess, a walled-off pocket of pus within the breast tissue. About 2 to 3% of mastitis cases progress to an abscess, with delayed treatment being a major contributing factor. An abscess feels like a firm, painful lump that doesn’t improve with antibiotics alone. It typically needs to be drained, either with a needle or a small incision, before it can heal.
The clearest way to lower your risk of complications is to address symptoms early. Mastitis that’s caught in the inflammatory stage and managed promptly with rest, frequent nursing, and ice rarely progresses to the point of needing antibiotics, let alone developing into an abscess. If you notice a red, painful area on your breast along with flu-like symptoms, that combination is the signal to act quickly rather than wait it out.