Oral candidiasis, also known as oral thrush among other names, is candidiasis that occurs in the mouth. That is, oral candidiasis is a mycosis (yeast/fungal infection) of Candida species on the mucous membranes of the mouth. Candida albicans is the most commonly implicated organism in this condition. C. albicans is carried in the mouths of about 50% of the world's population as a normal component of the oral microbiota. This candidal carriage state is not considered a disease, but when Candida species become pathogenic and invade host tissues, oral candidiasis can occur. This change usually constitutes an opportunistic infection by normally harmless micro-organisms because of local (i.e., mucosal) or systemic factors altering host immunity. Classification Oral candidiasis is a mycosis (fungal infection). Traditionally, oral candidiasis is classified using the Lehner system, originally described in the 1960s, into acute and chronic forms (see table). Some of the subtypes almost always occur as acute (e.g., acute pseudomembranous candidiasis), and others chronic. However, these typical presentations do not always hold true, which created problems with this system. A more recently proposed classification of oral candidiasis distinguishes primary oral candidiasis, where the condition is confined to the mouth and perioral tissues, and secondary oral candidiasis, where there is involvement of other parts of the body in addition to the mouth. The global human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) pandemic has been an important factor in the move away from the traditional classification since it has led to the formation of a new group of patients who present with atypical forms of oral candidiasis. By appearance Three main clinical appearances of candidiasis are generally recognized: pseudomembranous, erythematous (atrophic) and hyperplastic. Most often, affected individuals display one clear type or another, but sometimes there can be more than one clinical variant in the same person. Pseudomembranous Acute pseudomembranous candidiasis is a classic form of oral candidiasis, commonly referred to as thrush. Overall, this is the most common type of oral candidiasis, accounting for about 35% of oral candidiasis cases. It is characterized by a coating or individual patches of pseudomembranous white slough that can be easily wiped away to reveal erythematous (reddened), and sometimes minimally bleeding, mucosa beneath. These areas of pseudomembrane are sometimes described as "curdled milk", or "cottage cheese". The white material is made up of debris, fibrin, and desquamated epithelium that has been invaded by yeast cells and hyphae that invade to the