Thrush is caused by an overgrowth of Candida, a type of fungus that naturally lives on your skin, in your mouth, and throughout your digestive tract. Under normal conditions, your immune system and the bacteria sharing those same spaces keep Candida in check. When that balance breaks down, the fungus multiplies rapidly, shifts into a more aggressive form, and causes the white patches, soreness, and irritation recognized as thrush.
The Fungus Behind Thrush
Candida albicans is the species responsible for most thrush infections. It exists in two forms: a round, relatively harmless yeast cell and a thread-like form called hyphae. When conditions favor overgrowth, the fungus switches to the hyphal form and begins actively invading tissue. In this state, it secretes a toxin called candidalysin that damages the cells lining your mouth or other mucosal surfaces and triggers inflammation. The hyphae also produce sticky surface proteins that help the fungus anchor itself to tissue, making the infection harder for your body to clear on its own.
While C. albicans gets the most attention, other Candida species can also cause thrush. In one clinical study, C. albicans accounted for about 45% of identified Candida isolates, with C. glabrata making up roughly 20% and several other species filling in the rest. This matters because some non-albicans species respond differently to standard antifungal treatments. If a thrush infection doesn’t improve with typical therapy, a different Candida species may be the cause.
How Your Immune System Normally Prevents Thrush
Your body has a specific defense pathway dedicated to keeping Candida under control on mucosal surfaces like the inside of your mouth. A subset of immune cells called Th17 cells produces a signaling molecule, IL-17, that acts as the central coordinator of this defense. When Candida begins to overgrow, IL-17 signals the cells lining your mouth to release antimicrobial peptides that directly kill the fungus. It also recruits neutrophils, a type of white blood cell, to the site to help contain the infection.
This IL-17 pathway does double duty. Beyond fighting the fungus, it prevents excessive tissue damage during an immune response, keeping inflammation from spiraling out of control. When this pathway is weakened or absent, whether from disease, medication, or genetics, Candida faces little resistance and can proliferate freely. That’s why conditions affecting immune cell function are among the strongest risk factors for thrush.
Antibiotics and Bacterial Balance
One of the most common triggers for thrush is antibiotic use. Antibiotics kill bacteria, but they don’t distinguish between harmful bacteria and the beneficial species that compete with Candida for space and resources in your mouth and gut. When those protective bacteria are wiped out, Candida has room to expand unchecked.
The effect goes deeper than just clearing competitors. Research published in Cell Host & Microbe found that antibiotics impair the very immune cells responsible for antifungal defense. In animal models, antibiotic-driven disruption of gut bacteria reduced Th17 cell activity and weakened IL-17 signaling, the same pathway your body relies on to keep Candida contained. The result was not only fungal overgrowth but also a breakdown in gut barrier function that allowed bacteria to escape into the bloodstream. In humans, antibiotic use has been linked to an increased risk of both mucosal and invasive Candida infections. Broad-spectrum antibiotics, the kind prescribed for a wide range of bacterial infections, carry the highest risk.
Diabetes and Blood Sugar
People with diabetes, particularly those with poorly controlled blood sugar, develop thrush more frequently than the general population. The connection is straightforward: Candida feeds on sugar. When blood glucose runs high, glucose levels in saliva rise as well, creating an environment where the fungus thrives.
Research from the American Academy of Pediatric Dentistry found that in young people with insulin-dependent diabetes, higher average blood sugar levels correlated with higher yeast counts in the mouth. Those with poorer long-term glucose control (reflected in higher HbA1c values) had significantly more oral yeast than those with tighter control. While researchers haven’t identified an exact blood sugar threshold that flips the switch, the trend is consistent: the less controlled the diabetes, the more hospitable the mouth becomes for Candida. Reduced saliva flow, another common feature of diabetes, compounds the problem by washing away fewer fungal cells and lowering the concentration of protective proteins in saliva.
Other Conditions That Raise Your Risk
Any condition or treatment that suppresses immune function can set the stage for thrush. HIV is one of the most well-known examples, because the virus specifically destroys the T cells that coordinate antifungal defense. Oral thrush is often one of the earliest signs of advancing HIV infection. Cancer treatments, including chemotherapy and radiation to the head and neck, damage the mucosal lining and suppress immune cell production simultaneously. Organ transplant recipients taking immunosuppressive drugs face a similar vulnerability.
Inhaled corticosteroids, commonly used for asthma and COPD, are a frequently overlooked cause of oral thrush. The medication deposits directly on the tissues of the mouth and throat, suppressing the local immune response while leaving traces of steroid that encourage fungal growth. Rinsing your mouth with water after each use significantly reduces this risk. Dentures also create a favorable environment for Candida, especially if they fit poorly or aren’t cleaned daily, because the warm, moist space beneath the denture plate is ideal for fungal colonization.
Thrush in Babies and Breastfeeding
Oral thrush is especially common in newborns and young infants. Their immune systems are still developing, and the Th17 pathway that adults rely on to control Candida isn’t yet fully functional. Babies typically pick up Candida during birth or in the first weeks of life, and without a mature immune response to keep it in check, the fungus can overgrow quickly. The classic sign is white, cottage cheese-like patches on the tongue and inner cheeks that don’t wipe away easily.
Breastfeeding creates a cycle of reinfection between mother and baby. Candida from the baby’s mouth transfers to the nipple, where it can cause cracking, burning pain, and a shiny or flaky appearance on the skin. The fungus then passes back to the baby during the next feeding. Breaking this cycle usually requires treating both mother and baby at the same time, even if only one is showing symptoms. Sterilizing pacifiers, bottle nipples, and breast pump parts also helps interrupt transmission.
How Thrush Is Diagnosed
In most cases, oral thrush is diagnosed by its appearance alone. The white patches on a red, inflamed base are distinctive enough that a healthcare provider can identify it during a visual exam. When the diagnosis is uncertain, or when thrush keeps coming back despite treatment, a small sample may be taken from the affected area and examined under a microscope. This confirms the presence of Candida and can reveal whether the fungus is in its invasive hyphal form.
Global guidelines from infectious disease organizations still recommend microscopy and culture as the primary diagnostic tools for Candida infections. Culture is particularly useful when a non-albicans species is suspected, since identifying the exact species can guide treatment choices. Special staining techniques improve the ability to spot fungal elements in tissue samples. For recurrent or treatment-resistant thrush, especially in people with compromised immune systems, more advanced testing like PCR-based methods can identify the species with greater precision.