Vaginal bacterial infections happen when the balance of bacteria inside the vagina shifts, allowing harmful species to outnumber the protective ones. The most common type, bacterial vaginosis (BV), affects roughly 23 to 29% of women of reproductive age worldwide. It isn’t caused by a single germ entering the body the way a cold or flu might. Instead, it’s the result of conditions inside the vagina changing enough that bacteria already present in small numbers can multiply and take over.
What Happens Inside the Vagina
A healthy vagina is home to large colonies of bacteria called Lactobacilli. These bacteria produce lactic acid, which keeps the vaginal environment slightly acidic, typically at a pH below 4.5. That acidity acts like a natural defense system, making it difficult for harmful organisms to thrive.
When something disrupts those Lactobacilli colonies, the acid levels drop and the pH rises above 4.5, creating a more alkaline environment. Anaerobic bacteria (types that thrive without much oxygen) seize the opportunity. Species like Gardnerella vaginalis, Prevotella, Mobiluncus, and others that were previously a tiny minority in the vagina can multiply rapidly, sometimes forming a sticky film called a biofilm on the vaginal walls. This dramatic shift from protective bacteria to harmful ones is what produces the symptoms of BV: thin, grayish-white discharge with a noticeable fishy odor.
Sexual Activity and Partner Changes
BV occurs most often in people who are sexually active. Having a new sexual partner, or multiple partners, is one of the strongest risk factors. Sex introduces new bacteria into the vaginal environment, and semen itself is alkaline, which can temporarily raise vaginal pH. Over time or with frequent exposure, this creates conditions where harmful bacteria gain a foothold.
Not using condoms also increases the risk, likely because barrier protection limits direct contact with a partner’s genital bacteria and prevents semen from altering vaginal pH. BV is not classified as a sexually transmitted infection because it can occur in people who have never had sex, but sexual activity is clearly one of the most common triggers. Women who have sex with women can also pass BV-associated bacteria between partners.
Douching and Hygiene Products
Douching is one of the most well-documented causes of BV. Flushing water, vinegar, or commercial solutions into the vagina physically washes away Lactobacilli and disrupts the acidic environment they maintain. The vagina is self-cleaning, so douching doesn’t just fail to help; it actively removes the protective bacteria you need.
Scented soaps, bubble baths, and perfumed feminine hygiene products can have a similar effect. These products alter the chemistry of the vaginal area and can irritate the delicate tissue, making it easier for anaerobic bacteria to establish themselves. Washing the external genital area with plain warm water, or at most a mild unscented soap, is enough to stay clean without disrupting the internal balance.
Other Factors That Shift the Balance
Hormonal fluctuations play a role. Estrogen supports the growth of Lactobacilli, so any period of lower estrogen (around menstruation, during breastfeeding, or in perimenopause) can temporarily thin the protective bacterial layer. Menstrual blood itself is slightly alkaline, which is why some people notice BV symptoms appearing right after their period.
Antibiotics taken for unrelated infections can also wipe out vaginal Lactobacilli along with the targeted bacteria, leaving the door open for BV. Smoking has been linked to higher BV rates as well, though the exact mechanism isn’t fully understood. Intrauterine devices (IUDs) may slightly increase risk in some people, possibly by altering the local environment around the cervix.
How BV Is Recognized
The hallmark symptom is a thin, milky discharge that coats the vaginal walls and often carries a fishy smell. The odor tends to be stronger after sex or during a period. Some people also experience mild itching or burning, though many have no symptoms at all and only discover BV during a routine exam.
Clinicians typically look for at least three of the following four signs: the characteristic thin discharge, a vaginal pH above 4.5, the presence of “clue cells” (vaginal cells covered in a layer of bacteria visible under a microscope), and a fishy odor when the discharge is exposed to a chemical solution. A lab test called a Nugent score, which grades a sample of vaginal fluid under a microscope, can also confirm the diagnosis.
Why It Matters Beyond Discomfort
BV is more than an inconvenience. The shift to an alkaline, bacteria-heavy environment weakens the vagina’s natural defenses against other infections. People with BV are more susceptible to sexually transmitted infections, including chlamydia, gonorrhea, and HIV, because the protective Lactobacilli barrier is compromised. During pregnancy, untreated BV has been linked to preterm birth and low birth weight, which is why screening may be recommended for pregnant people with a history of preterm delivery.
BV can also increase the risk of infection after gynecological procedures like IUD insertion or hysterectomy, since the altered bacterial environment provides less protection during recovery.
Treatment and Recurrence
BV is treated with prescription antibiotics, either taken orally or applied as a vaginal gel or cream. Most courses last five to seven days, and symptoms usually improve within a few days of starting treatment. The frustrating part is recurrence: roughly half of people treated for BV experience it again within 12 months. The biofilm that harmful bacteria form on vaginal walls is difficult to fully eliminate, and reexposure to the same triggers (a sexual partner carrying BV-associated bacteria, douching, or hormonal shifts) can restart the cycle.
Probiotics containing Lactobacillus strains, particularly Lactobacillus rhamnosus, Lactobacillus reuteri, and Lactobacillus crispatus, have been studied as a way to reduce recurrence. A meta-analysis of randomized controlled trials found that probiotics after antibiotic treatment can help lower recurrence rates, though the optimal strain, dose, and duration are still being refined. Probiotics are available orally and as vaginal suppositories, and they appear most promising when used alongside standard antibiotic therapy rather than as a replacement.
Practical Steps to Lower Your Risk
- Skip douching entirely. Your vagina maintains its own bacterial ecosystem without help.
- Use condoms with new or multiple partners. Barrier protection limits bacterial transfer and shields the vagina from alkaline semen.
- Avoid scented products near the vagina. This includes perfumed soaps, sprays, and scented tampons or pads.
- Wear breathable cotton underwear. Moisture-wicking fabrics keep the area dry, which discourages anaerobic bacterial growth.
- Change out of wet clothing promptly. Sitting in a damp swimsuit or sweaty workout clothes creates a warm, moist environment where harmful bacteria thrive.
- Consider probiotics if BV keeps returning. Talk to your provider about whether a Lactobacillus-based probiotic after treatment could help break the cycle.