Candida yeast cells. Coloured scanning electron micrograph (SEM) of Candida sp. cells. Candida is a genus of yeasts and is the most common cause of fungal infections worldwide. It is the largest genus of medically important yeasts. The genus Candida encompasses about 200 species. Many species are harmless commensals or endosymbionts of hosts including humans; however, when mucosal barriers are disrupted or the immune system is compromised they can invade and cause disease, known as an opportunistic infection. Candida is located on most mucosal surfaces and mainly the gastrointestinal tract, along with the skin. Recently Candida auris, which affects primarily older people with weakened immune systems, has become a serious problem and is particularly dangerous because it resists treatment by common antifungal medications. Magnification: x4000 when printed 10 centimetres wide.