Thrush is caused by the overgrowth of a fungus called candida.
We all have many germs living on or inside us all the time. Some of these germs are helpful to us - for instance, 'friendly' bacteria in the gut help us to digest our food. Many others don't cause any harm, or any symptoms, unless they grow too much. Candida is one of these.
This is a companion discussion topic for the original article at https://patient.info/sexual-health/vaginal-discharge-female-discharge/vaginal-thrush-yeast-infection