Yeast refers to single-celled microscopic fungi, most commonly species of the genus Candida (especially Candida albicans) or Saccharomyces (e.g., baker’s/brewer’s yeast), that can live harmlessly on human skin, in the mouth, gut, vagina, or digestive tract as part of the normal microbiome. In certain conditions—such as antibiotic use (which kills competing bacteria), high sugar levels (e.g., diabetes), hormonal changes (pregnancy, birth control), weakened immunity (HIV, chemotherapy, corticosteroids), moist environments, or poor hygiene—yeast can overgrow and cause infections known as candidiasis or thrush. Common manifestations include oral thrush (white patches in the mouth), vaginal yeast infections (intense itching, thick white discharge, redness), cutaneous candidiasis (red, itchy rashes in skin folds like under breasts, groin, or armpits), or invasive candidiasis (serious bloodstream infection in immunocompromised people). Yeast overgrowth often produces symptoms like itching, burning, soreness, creamy or cottage-cheese-like discharge/patches, and sometimes a yeasty odor, with affected areas appearing red, inflamed, or raw.