Thrush is a yeast infection that is caused by the candida species of fungus. Thrush is also called candida or candidosis. It happens when a fungus called Candida albicans multiplies.
Candida occurs naturally in the body, particularly in warm, moist areas such as the mouth and genitals. It normally doesn't cause any problems because it's kept under control by the immune system and other bacteria in the body. However, some factors can cause the fungus to multiply (grow), leading to infection.
Thrush is not a sexually transmitted infection
Thrush is not a sexually transmitted infection (STI) because many people already have a small amount of candida in their bodies. In fact, the infection is actually more common in men who are not sexually active.