Yeast already lives on and inside your body. The fungus Candida, the type responsible for most yeast infections, naturally colonizes your mouth, gut, and skin without causing problems. You “get” a yeast infection when something disrupts the balance that keeps that fungus in check, allowing it to multiply and invade tissue. Up to 75% of women will experience at least one vaginal yeast infection in their lifetime, and infections of the mouth, skin, and other areas are common across all ages and sexes.
If you’re looking for yeast in the culinary sense, for baking or brewing, the answer is simpler: you buy it at a grocery store, or you capture wild yeast from the environment. Both paths are covered below.
What Triggers a Yeast Infection
Candida normally lives alongside bacteria that limit its growth. When those bacteria are killed off or your immune system weakens, Candida shifts from a harmless passenger to an active invader. The fungus changes shape, growing long filaments that can physically penetrate tissue and anchor into the lining of your mouth, vagina, or gut. It also builds protective layers called biofilms and becomes highly efficient at scavenging iron, sugar, and other nutrients from your body.
Several common situations set this process in motion.
Antibiotics
Antibiotics are the single most recognized trigger. They kill the protective bacteria that normally compete with yeast for space and resources, leaving Candida free to expand. Not all antibiotics carry equal risk. A large study found that azithromycin more than doubled the risk of a vaginal yeast infection after treatment, while doxycycline raised the risk by about 24%. Other antibiotics like minocycline and erythromycin did not show a statistically significant increase. The broader the antibiotic’s reach, the more bacterial species it wipes out, and the easier it becomes for yeast to fill the gap.
High Blood Sugar
Diabetes is a major risk factor. When blood sugar stays elevated, excess glucose shows up in urine, vaginal secretions, and saliva, essentially feeding Candida a steady supply of its preferred fuel. People with poorly controlled diabetes are significantly more prone to vaginal, oral, and skin yeast infections. Even without diabetes, temporary blood sugar spikes from illness or steroid medications can create a similar environment.
Weakened Immunity
Anything that suppresses your immune system gives Candida an opening. HIV, cancer treatment, organ transplant medications, and chronic corticosteroid use all reduce your body’s ability to keep the fungus contained. In severe cases, Candida can move from the gut into the bloodstream, causing a dangerous systemic infection. This is rare in healthy people but is a real concern for hospitalized or immunocompromised patients.
Medications Beyond Antibiotics
Inhaled corticosteroids for asthma are a well-known cause of oral thrush (a yeast infection of the mouth). The medication suppresses the local immune response in the throat and mouth. Medications that cause dry mouth also raise risk, since saliva contains antifungal compounds. Dentures create another favorable environment, trapping moisture against the tissue.
Moisture, Clothing, and Everyday Habits
Yeast thrives in warm, moist environments. Synthetic fabrics like nylon and spandex trap heat and moisture against the skin, creating ideal conditions for Candida to multiply. Cotton underwear is breathable and absorbent, which helps keep the area dry. For workouts, moisture-wicking fabrics are a better choice than standard synthetics.
Sitting in a wet swimsuit, wearing tight clothing for long periods, or sleeping in underwear can all contribute. Going without underwear overnight lets the area breathe and reduces moisture buildup. These factors alone rarely cause an infection, but they can tip the balance when other risk factors are already present.
Can You Catch a Yeast Infection From Someone Else?
Yeast infections are not classified as a sexually transmitted infection because plenty of people who have never had sex get them, including babies and children. That said, the fungus can be passed between partners. About 15% of men develop symptoms of a penile yeast infection after sex with an infected female partner. Transmission can also go the other direction, from a male partner to a female one. Oral sex with someone who has an active vaginal or penile yeast infection can lead to oral thrush, and Candida can even be shared through kissing.
None of this means you need to treat a yeast infection like a contagious disease. Most people already carry Candida. Transmission from a partner just adds to the fungal load, which only becomes a problem if local conditions favor overgrowth.
Yeast Infections in Babies
Babies commonly develop yeast-related diaper rash, and it looks different from ordinary diaper irritation. A standard diaper rash tends to appear as a single dry, scaly, pinkish patch on the buttocks. A yeast diaper rash shows up in skin folds near the groin, legs, and genitals. The skin looks bumpy, shiny, and sometimes cracked or oozy, with a deeper red or purple tone. It often appears in several smaller spots rather than one large area.
Regular diaper cream won’t clear a yeast rash. It requires antifungal treatment and can take a few weeks to fully resolve, compared to a couple of days for standard irritation. If a diaper rash isn’t improving with normal care, the yeast version is a likely explanation.
Oral Thrush Risk Factors
Oral thrush produces white patches on the tongue, inner cheeks, and roof of the mouth. Beyond inhaled corticosteroids and dry mouth medications, smoking is an independent risk factor. Dentures that fit poorly or aren’t cleaned regularly also promote growth. Infants and older adults are most susceptible because their immune responses are either immature or declining.
Drug-Resistant Yeast: Candida Auris
A species called Candida auris has emerged as a serious concern in healthcare settings. Unlike typical Candida infections, C. auris resists many standard antifungal medications and can survive on surfaces for weeks. The CDC reported 6,304 clinical cases in the U.S. in 2024 alone, with more than 17,000 clinical cases tallied since tracking began in 2016. An additional 35,000-plus screening cases have been identified in patients who carry the fungus without active symptoms. This species primarily affects people already in hospitals or long-term care facilities. It is not a common cause of everyday yeast infections in healthy people.
Getting Yeast for Baking and Brewing
If your search is about the kitchen rather than the clinic, commercial baker’s yeast (Saccharomyces cerevisiae) is sold in grocery stores as active dry yeast, instant yeast, or fresh cake yeast. All three are the same species, just processed differently. Active dry yeast needs to be dissolved in warm water before use. Instant yeast can go directly into dry ingredients. Fresh yeast is perishable and sold in refrigerated blocks.
You can also capture wild yeast from the environment to make a sourdough starter. Wild yeast lives on grain, in flour, on fruit skins, on your hands, and floating in the air of your home. To collect it, mix equal parts flour and dechlorinated water in a jar (filtered water works, or tap water left out overnight to off-gas chlorine). Cover the jar with a paper towel secured by a rubber band, which keeps out debris while letting airborne microbes drift in. Place the jar somewhere warm and out of direct sunlight. Feed the mixture daily with fresh flour and water. Within five to fourteen days, wild yeast and beneficial bacteria will colonize the starter, producing the bubbles and sour smell that signal it’s active. Whole grain flours like rye and whole wheat tend to jump-start the process faster because they carry more wild yeast on the grain’s outer layers.