Candida albicans is a type of fungus that naturally lives in and on the human body. It resides in the mouth, gut, reproductive tract, and on the skin of most healthy people without causing any harm. Problems start only when something disrupts the body’s normal balance, allowing the fungus to multiply beyond what the immune system can keep in check. That overgrowth is what causes the range of infections collectively known as candidiasis.
A Shapeshifting Fungus
What makes Candida albicans unusual, and particularly good at causing infections, is its ability to change shape. In its harmless state, it grows as round, single-celled yeast. But when conditions shift, it can rapidly transform into long, thread-like filaments called hyphae. These filaments can physically penetrate tissue, helping the fungus invade the lining of the mouth, gut, or vaginal walls. A standard trigger for this switch is a combination of body temperature (37°C) and the presence of certain proteins in blood serum.
Beyond these two main forms, C. albicans can also grow as pseudohyphae (elongated cells that stay linked in chains), form thick-walled survival spores called chlamydospores under harsh conditions, and switch into a flattened “opaque” cell type with a distinctly pitted surface. This morphological flexibility helps it adapt to wildly different environments throughout the body.
Where It Lives in Healthy People
C. albicans is a commensal organism, meaning it coexists with you without causing disease under normal circumstances. It colonizes the gastrointestinal tract from the mouth all the way through the intestines, the vaginal lining, and the skin surface. Different populations of the fungus express different surface proteins depending on where they live, essentially tailoring themselves to each niche. In a healthy person with an intact immune system, bacteria and immune cells keep Candida populations small and contained.
What Triggers Overgrowth
Candida infections happen when the balance tips in the fungus’s favor. The most common triggers fall into a few categories:
- Antibiotics: These kill off the bacteria that normally compete with Candida for space and nutrients, especially in the gut and vagina. Broad-spectrum or long-term antibiotic use carries the highest risk.
- Weakened immunity: HIV/AIDS, cancer, chemotherapy, and organ transplant medications all reduce the body’s ability to suppress fungal growth.
- Diabetes: Elevated blood sugar provides extra fuel for Candida and impairs immune function. Diabetes is a risk factor for oral, vaginal, and even bloodstream Candida infections.
- Hormonal changes: Pregnancy and hormonal birth control increase the risk of vaginal yeast infections, likely due to changes in vaginal acidity and sugar content.
- Corticosteroids: Inhaled steroids used for asthma are a well-known trigger for oral thrush, as are oral steroids that suppress immune activity.
- Dry mouth: Saliva helps control oral Candida. Medications or conditions that reduce saliva flow raise the risk of mouth and throat infections.
How It Causes Infection
C. albicans has several biological tools that make it an effective pathogen once conditions allow overgrowth. It secretes enzymes that break down host cell membranes, allowing it to attach to and invade mucosal surfaces and even blood vessel walls. These same enzymes help it evade immune detection.
The fungus also forms biofilms, structured communities of cells embedded in a protective matrix. Biofilms can develop on living tissue or on medical devices like catheters and implants. Once a biofilm is established, it becomes far more resistant to both antifungal drugs and immune responses. Most serious Candida infections involve biofilm formation.
Common Types of Candida Infection
Oral Thrush
Thrush appears as white patches on the inner cheeks, tongue, roof of the mouth, and throat. You may notice redness, soreness, a cotton-like feeling in the mouth, loss of taste, pain while eating or swallowing, and cracking at the corners of the mouth. It’s most common in people with weakened immune systems, diabetes, or those using inhaled corticosteroids.
Vaginal Yeast Infections
Vaginal candidiasis is extremely common. Symptoms include itching or soreness, pain during sex or urination, and abnormal discharge. Severe cases can involve redness, swelling, and cracks in the vaginal wall. About 75% of women experience at least one vaginal yeast infection in their lifetime, and recurrent infections (four or more per year) affect a smaller but significant group.
Esophageal Candidiasis
When the infection spreads deeper into the esophagus, it causes pain and difficulty swallowing. Most people with esophageal candidiasis also have visible thrush in their mouth and throat. This form is more common in people with advanced immune suppression.
Invasive Candidiasis
The most dangerous form occurs when Candida enters the bloodstream (candidemia) and spreads to internal organs. This typically happens in hospitalized patients, particularly those with central venous catheters, recent surgery, or prolonged antibiotic use. A multinational study including 463 patients found that overall mortality from invasive Candida infections was 39.4%, making it one of the most lethal hospital-acquired fungal infections.
How Candida Infections Are Diagnosed
Diagnosis depends on the location of the infection. For vaginal yeast infections, a provider takes a small sample of discharge and examines it under a microscope or sends it for a fungal culture. Oral thrush is often diagnosed by visual examination alone, though a swab can be sent to the lab for confirmation. Esophageal candidiasis requires endoscopy, where a small camera is passed down the throat to directly view the infection. Invasive candidiasis is diagnosed through blood cultures, where a blood sample is checked to see if Candida grows in the lab.
Treatment for Different Infections
Most Candida infections respond well to antifungal medications, with the specific approach depending on the infection’s location and severity.
Mild oral thrush is typically treated with antifungal lozenges or tablets applied directly in the mouth for 7 to 14 days. Moderate to severe cases require an oral antifungal taken for the same duration. Vaginal yeast infections can often be cleared with over-the-counter antifungal creams or suppositories, or a single oral antifungal dose. Treatment with these medications resolves symptoms and clears the infection in 80% to 90% of patients who complete therapy.
Recurrent vaginal infections require a different strategy: an initial treatment course of 7 to 14 days followed by a weekly oral antifungal for six months to prevent relapse. During pregnancy, only topical antifungal creams applied for seven days are recommended, since oral antifungals have been linked to complications.
Invasive candidiasis requires intravenous antifungal therapy in a hospital setting, typically with a class of drugs called echinocandins as the first choice.
Growing Resistance to Antifungals
C. albicans has historically been one of the more treatable Candida species, but resistance is becoming a concern. Overall, fewer than 5% of vaginal C. albicans isolates show resistance to fluconazole (the most commonly prescribed oral antifungal) under standard lab conditions. However, a 10-year study at a clinical referral center found resistance in 23% of isolates that were specifically sent for testing, likely because providers request testing when patients aren’t responding to treatment.
More troubling, the vaginal environment is naturally acidic, and when researchers tested resistance at the lower pH that actually exists in the vagina, resistance rates jumped to 52%. This suggests that standard lab testing, which is done at a neutral pH, may underestimate how well the fungus can resist treatment in real-world conditions. Resistance rates at the referral center also increased over time, rising from around 15% to 36% between 2014 and 2021 under standard testing conditions.
For infections that don’t respond to first-line treatment, alternative antifungal classes are available, but the trend underscores why completing a full course of antifungal therapy matters and why recurrent infections deserve a closer look rather than repeated rounds of the same medication.