It is a fungus - a fungus gets its nourishment by sending tendrils into its food and prying the cells apart - this stretches the tissue and sets off the nerve endings most fungi are saprophytes (they eat dead stuff)

1) "Candidiasis, commonly called yeast infection or thrush, is a fungal infection (mycosis) of any of the Candida species, of which Candida albicans is the most common. Candidiasis thereby encompasses infections that range from superficial, such as oral thrush and vaginitis, to systemic and potentially life-threatening diseases. Candida infections of the latter category are also referred to as candidemia and are usually confined to severely immunocompromised persons, such as cancer, transplant, and AIDS patients, whereas superficial infections of skin and mucosal membranes by Candida causing local inflammation and discomfort is common in many human populations. While clearly attributable to the presence of the opportunistic pathogens of the genus Candida, candidiasis describes a number of different disease syndromes that often differ in their causes and outcomes" Source and further information: http://en.wikipedia.org/wiki/Candidiasis 2) "Mechanism The central chemical involved in itching is histamine, a molecule released by mast cells in the skin. Histamine is the chemical that causes the itch and reddening. It binds to local nerve endings on specific receptors. An itch from cutaneous (skin-related) stimuli, such as movement of small hairs on the body, was thought to be transmitted along the same pathway as pain, although recent research has found that itching has its own neural pathways. An itch caused by histamine is transmitted to the brain by a different neural pathway, described in 2001 by Andrew and Craig. As with pain, a histamine-induced itch travels via the spinothalamic tract, but in fibres specific for itch." "The feeling of itchiness can be caused by a movement of hair or the release of a chemical (histamine) from cells under the skin. Itchiness is regarded as protective, as it helps creatures remove parasites that land on their skin." Source and further information: http://en.wikipedia.org/wiki/Itching