lesions - the result of invasion of the host tissues - this is termed candidiasis. Some consider oral candidiasis a change in the normal oral environment rather than an exposure or true "infection" as such. The exact process by which Candida species switch from acting as normal oral commensals (saprophytic) state in the carrier to acting as a pathogenic organism (parasitic state) is not completely understood. Several Candida species are polymorphogenic, that is, capable of growing in different forms depending on the environmental conditions. C. albicans can appear as a yeast form (blastospores), which is thought to be relatively harmless; and a hyphal form associated with invasion of host tissues. Apart from true hyphae, Candida can also form pseudohyphae — elongated filamentous cells, lined end to end. As a general rule, candidiasis presenting with white lesions is mainly caused by Candida species in the hyphal form and red lesions by yeast forms. C. albicans and C. dubliniensis are also capable of forming germ tubes (incipient hyphae) and chlamydospores under the right conditions. C. albicans is categorized serologically into A or B serotypes. The prevalence is roughly equal in healthy individuals, but type B is more prevalent in immunocompromised individuals. Predisposing factors The host defenses against opportunistic infection of candida species are The oral epithelium, which acts both as a physical barrier preventing micro-organisms from entering the tissues, and is the site of cell mediated immune reactions. Competition and inhibition interactions between candida species and other micro-organisms in the mouth, such as the many hundreds of different kinds of bacteria. Saliva, which possesses both mechanical cleansing action and immunologic action, including salivary immunoglobulin A antibodies, which aggregate candida organisms and prevent them adhering to the epithelial surface; and enzymatic components such as lysozyme, lactoperoxidase and antileukoprotease. Disruption to any of these local and systemic host defense mechanisms constitutes a potential susceptibility to oral candidiasis, which rarely occurs without predisposing factors. It is often described as being "a disease of the diseased", occurring in the very young, the very old, or the very sick. Immunodeficiency Immunodeficiency is a state of reduced function of the immune system, which can be caused by medical conditions or treatments. Acute pseudomembranous candidiasis occurs in about 5% of newborn infants. Candida species are acquired from the mother's vaginal canal during birth. At very young ages, the immune system is yet to develop fully and there is no