Yeast infections happen when a fungus called Candida, which normally lives in small amounts on your skin and inside your body, multiplies out of control. Up to 75% of women will experience at least one vaginal yeast infection in their lifetime. The fungus is almost always already present; what changes is the environment that keeps it in check.
Candida Already Lives in Your Body
Candida is part of your normal microbiome. It exists on your skin, in your mouth, in your gut, and in the vaginal tract without causing problems. In its inactive state, Candida takes a round, single-celled yeast form. It only becomes harmful when conditions shift and the organism transitions into a more aggressive, thread-like form that can penetrate tissue and spread. This shape-shifting ability is what separates a harmless passenger from an active infection.
That transition is tightly regulated by internal signals within the fungus itself, including its own metabolic state and chemical messengers like nitric oxide. When the surrounding environment provides the right cues, those internal brakes release, and the fungus begins to grow rapidly and invade the tissue it once coexisted with peacefully.
The Protective Role of Vaginal Bacteria
In the vaginal tract, the main line of defense against Candida overgrowth is a group of bacteria called Lactobacillus. These bacteria break down glycogen (a sugar stored in the cells lining the vaginal wall) and convert it into lactic acid. This keeps vaginal pH between 3.5 and 4.5, an acidic range that favors Lactobacillus and suppresses competing organisms, including yeast.
Lactobacillus species do more than just produce acid. They generate hydrogen peroxide, which damages organisms that lack the enzymes to neutralize it. They produce antimicrobial peptides that directly kill or inhibit pathogens. And they physically crowd out Candida by occupying the surface of vaginal tissue, blocking yeast from attaching to cell walls. When this bacterial community is healthy and abundant, Candida stays in its harmless form and at low numbers. When it’s disrupted, the door opens for overgrowth.
Antibiotics Are a Common Trigger
Broad-spectrum antibiotics are one of the most well-known triggers for yeast infections. These medications are designed to kill bacteria, and they don’t distinguish between harmful bacteria causing your sinus infection and the protective Lactobacillus in your vaginal tract. Without enough Lactobacillus, vaginal acidity drops, and yeast finds an environment where it can thrive. A yeast infection can develop during or shortly after finishing a course of antibiotics.
This doesn’t mean you should avoid antibiotics when you need them. But it does explain why many women notice a pattern of yeast infections following treatment for something entirely unrelated, like a urinary tract infection or strep throat.
How Hormones Feed Yeast Growth
Estrogen plays a direct role in creating conditions that favor Candida. Higher estrogen levels increase the amount of glycogen deposited in vaginal tissue. More glycogen means more fuel for both Lactobacillus and, if the balance tips, for yeast. Estrogen also changes the structure of the vaginal lining in ways that make it easier for Candida to adhere to cells and establish itself.
This is why yeast infections are more common during pregnancy, when estrogen levels are significantly elevated. It also explains patterns some women notice around hormonal birth control use or during certain phases of the menstrual cycle. Before puberty and after menopause, when estrogen levels are low, vaginal yeast infections are relatively uncommon.
Blood Sugar and Yeast
Yeast feeds on sugar. When blood sugar levels run high, glucose concentrations in vaginal secretions also rise, giving Candida a rich carbon source to fuel its growth. Elevated blood sugar also shifts vaginal pH, making the environment more hospitable to yeast. This is why women with poorly controlled diabetes are significantly more likely to experience recurrent yeast infections. Getting blood sugar under better control often reduces the frequency of infections.
Other Factors That Shift the Balance
Several other situations can tip the balance in Candida’s favor:
- A weakened immune system. Your immune cells actively patrol the vaginal lining and suppress Candida growth. Conditions or medications that reduce immune function, like HIV or immunosuppressive drugs, remove that surveillance.
- Moisture and heat. Yeast thrives in warm, damp environments. Tight synthetic clothing, wet swimsuits worn for hours, or non-breathable underwear can create conditions that encourage overgrowth on the skin and in the vaginal area.
- Douching and harsh products. Washing inside the vaginal canal or using scented soaps disrupts the Lactobacillus community and changes pH. The vagina is self-cleaning; introducing outside products tends to cause more problems than it solves.
- Stress and fatigue. Chronic stress suppresses immune function and can alter hormonal balance, both of which affect the vaginal environment indirectly.
Why Some People Get Recurring Infections
Recurrent vulvovaginal candidiasis, defined as four or more infections in a single year, affects an estimated 138 million women globally each year. For many of these women, no single obvious trigger explains the pattern. Some people appear to have a genetic predisposition: their immune systems respond less aggressively to Candida, allowing small colonies to re-establish themselves after treatment clears the initial infection.
In other cases, the Candida never fully leaves. It can form protective communities on vaginal tissue that resist antifungal treatment, surviving at low levels and flaring back up when conditions shift. Women with recurrent infections often cycle through the same triggers repeatedly: a round of antibiotics, a hormonal shift, a stressful period at work. Identifying and addressing personal patterns can help reduce flare-ups, though some women need longer or maintenance-level treatment to keep the fungus suppressed.
What a Yeast Infection Feels Like
The hallmark symptoms are intense itching and irritation around the vulva and vaginal opening, along with a thick, white discharge that’s often compared to cottage cheese. The discharge typically doesn’t have a strong odor, which is one way to distinguish a yeast infection from bacterial vaginosis, where a fishy smell is common. You might also notice redness, swelling, burning during urination, or pain during sex.
One useful diagnostic clue: yeast infections occur at a normal vaginal pH (below 4.5), while bacterial vaginosis and other infections tend to raise pH above that range. This is part of why the two conditions feel different and require different treatments. Over-the-counter antifungal creams and suppositories treat yeast but won’t help bacterial vaginosis, and antibiotics for bacterial vaginosis can actually trigger a yeast infection. If you’re unsure which you’re dealing with, especially if it’s your first time or symptoms don’t match the typical pattern, getting tested gives you a clear answer and the right treatment.