Candidiasis, commonly called a yeast infection or thrush, is an infection caused by fungi in the genus Candida. These organisms live normally on skin and mucous membranes but can become pathogenic when local or systemic defenses change. Clinically it appears as superficial overgrowth of yeast in the mouth, genital tract or skin, and in vulnerable patients it can invade the bloodstream or internal organs. The condition is part of a wider category of fungal infection and is considered an infectious disease when symptomatic.

Types and common presentations

Superficial forms include oral thrush, vulvovaginal candidiasis, and cutaneous candidiasis. Symptoms vary by site but often include white patches or discharge, redness, itching, soreness and discomfort. In hospitalized or immunocompromised patients, invasive candidiasis (candidemia) can cause fever, organ dysfunction and requires urgent care.

Causes, risk factors and biology

Candida species, especially Candida albicans, can switch between harmless and invasive behavior. Risk factors for overgrowth include broad‑spectrum antibiotics, corticosteroids, diabetes, pregnancy, impaired immunity (HIV, chemotherapy), indwelling catheters and breaches of skin or mucous barriers. Biofilm formation on medical devices contributes to persistence and resistance.

Diagnosis

Diagnosis combines clinical signs with laboratory tests: microscopic examination with potassium hydroxide, culture, antigen or nucleic acid tests for mucosal disease, and blood cultures or molecular assays for suspected invasive infection. Identifying the species helps guide therapy because some non‑albicans species show reduced susceptibility to common antifungals.

Treatment and prevention

  • Topical antifungals (azoles, nystatin) are effective for many superficial infections.
  • Oral azoles (e.g., fluconazole) are commonly used for more extensive mucosal disease.
  • Invasive infections often require systemic therapy with echinocandins or amphotericin B, guided by species and susceptibility.
  • Prevention focuses on reducing risk factors, device management, glycemic control and prudent antibiotic use.

Understanding the differences between colonization and invasive disease, and between Candida species, is important for appropriate management. Candidiasis is a common clinical problem with outcomes that range from easily treated superficial disease to severe, life‑threatening infection in high‑risk patients.