Yeast infections are caused by an overgrowth of a fungus called Candida, most commonly the species Candida albicans. This fungus already lives on your skin and mucous membranes as part of your body’s normal microbial community. It only becomes a problem when something disrupts the balance that keeps it in check, allowing it to multiply and shift into a more aggressive form. An estimated 75% of women will experience at least one vaginal yeast infection in their lifetime, and 40% to 45% will have two or more episodes.
How a Harmless Fungus Becomes an Infection
Candida albicans is a lifelong resident of the human body. In small numbers, it coexists peacefully with the bacteria and immune cells that keep it under control. But when conditions change, the fungus can switch from a round, inactive yeast form into long, thread-like filaments called hyphae. This shape change is one of its most important tricks. In its filament form, Candida can physically penetrate tissue, secrete enzymes that break down cells, and form sticky biofilms that are harder for your immune system to clear.
The fungus is remarkably good at sensing its environment. It can detect changes in pH, temperature, and available nutrients, then adjust its behavior accordingly. It even actively alters the chemistry around it, releasing compounds that make the surrounding environment more alkaline, which further promotes its shift into that aggressive filament form.
Antibiotics and Bacterial Balance
One of the most common triggers for a yeast infection is a course of antibiotics. Your body relies on beneficial bacteria, particularly Lactobacillus species, to keep Candida in check. These bacteria compete with yeast for space and resources, and they help maintain an acidic environment that discourages fungal growth. Broad-spectrum antibiotics don’t distinguish between harmful and helpful bacteria. They wipe out protective Lactobacillus populations along with everything else.
The damage goes beyond just removing bacterial competition. Antibiotics also weaken a specific arm of the immune system in the gut. Certain beneficial bacteria are needed to train and activate immune cells that produce a signaling molecule critical for fighting fungal invaders. When antibiotics eliminate those bacteria, the immune response against Candida drops significantly. Animal studies have shown that antibiotic-exposed subjects had markedly increased fungal growth across the entire gastrointestinal tract compared to those with intact bacterial communities.
Hormones and Blood Sugar
Estrogen plays a direct role in creating conditions that favor yeast. Higher estrogen levels cause vaginal cells to deposit more glycogen, a form of stored sugar. This glycogen serves as fuel for both Lactobacillus bacteria and Candida. Under normal circumstances, Lactobacillus dominates and converts glycogen into lactic acid, keeping the vaginal pH around 4.0 and suppressing yeast. But if that bacterial balance is already disrupted, the extra glycogen becomes a buffet for Candida instead.
This is why yeast infections are more common during pregnancy, in the second half of the menstrual cycle, and in people taking hormonal birth control that raises estrogen levels. All of these situations increase glycogen availability in vaginal tissue.
Uncontrolled diabetes creates a similar problem through a different route. When blood sugar runs high, glucose levels rise in saliva, vaginal secretions, and skin surfaces. Candida thrives on this excess sugar. Elevated blood glucose also raises glycogen levels in vaginal tissue, which drops the local pH in ways that paradoxically favor Candida colonization. On top of that, high blood sugar impairs the ability of white blood cells to kill yeast cells effectively, giving the fungus a double advantage.
Weakened Immune Defenses
Your immune system is the primary force holding Candida in its harmless form. Anything that suppresses immune function can open the door to infection. People taking corticosteroids, anti-rejection drugs after organ transplants, or undergoing chemotherapy face higher risk. HIV infection, particularly when it depletes certain immune cells, is another well-established risk factor.
Even temporary immune suppression matters. Stress, sleep deprivation, and illness can all reduce your body’s ability to police fungal populations. Long hospital stays independently increase risk, partly because of the combination of antibiotics, invasive devices, and immune strain that hospitalization involves.
Moisture, Heat, and Clothing
Candida grows best in warm, moist environments with limited airflow. On the skin, infections tend to develop in creased areas like the armpits, groin, and beneath the breasts, where sweat collects and skin touches skin. Tight, non-breathable clothing traps heat and moisture against the body, creating ideal conditions for fungal overgrowth. Synthetic underwear, wet swimsuits left on too long, and occlusive workout gear are common culprits.
Diaper rash in infants is often a Candida skin infection for the same reason. The warm, sealed environment inside a diaper provides exactly the conditions the fungus needs.
Yeast Infections in Men
Men get yeast infections too, most commonly on the head of the penis, a condition called candidal balanitis. The most frequent cause is inadequate hygiene in uncircumcised men. The foreskin creates a warm, moist space with limited air exposure where urine, discharge, and a waxy substance called smegma can accumulate. Candida albicans is a normal resident of penile skin, but this combination of warmth, moisture, and trapped irritants allows it to shift from harmless colonizer to active infection.
While balanitis isn’t classified as a sexually transmitted infection, the organisms responsible can pass between partners. Men whose sexual partners have recurrent vaginal yeast infections face a higher risk. Diabetes and pH changes on the skin also increase susceptibility in the same way they do elsewhere on the body.
Less Common Candida Species
Candida albicans causes the majority of yeast infections, but it’s not the only species involved. Candida glabrata ranks second or third as a cause of both superficial and systemic infections. Other species include Candida tropicalis, Candida parapsilosis, and Candida krusei. These alternatives matter because some of them respond differently to standard antifungal treatments.
Candida glabrata behaves differently from albicans in notable ways. It doesn’t form the same thread-like filaments and appears less sensitive to environmental changes that trigger albicans infections. Vaginal infections caused by glabrata tend to occur at a slightly higher pH than albicans infections. Prior antibiotic use and prolonged hospitalization are risk factors for non-albicans infections, similar to albicans, but these species are increasingly recognized in people who have had multiple rounds of antifungal treatment.
How Yeast Infections Differ From Other Vaginal Infections
One useful distinction is pH. A vaginal yeast infection typically occurs at a normal vaginal pH of around 4.0. Bacterial vaginosis, by contrast, pushes pH above 4.5 because the overgrowth of anaerobic bacteria replaces the acid-producing Lactobacillus. Trichomoniasis, a parasitic infection, drives pH even higher, often above 5.4. This is why a simple pH test can help distinguish between these conditions, though it’s not foolproof on its own.
The symptoms also differ. Yeast infections produce a thick, white, clumpy discharge with no strong odor, along with itching, redness, and swelling. Bacterial vaginosis typically causes a thin, grayish discharge with a fishy smell but little inflammation. These patterns aren’t always clear-cut, which is why infections that don’t respond to over-the-counter treatment or keep coming back deserve a closer look.