Hi.
According to Dr. Karen Rosewater, thrush is caused by a fungus called Candida albicans, which is a form of yeast that normally exists in the human digestive system, skin and the mouth.
In a healthy person, the bacteria and other microorganisms present in the body are enough to keep the yeast in check, but stress, certain illnesses, or medications can sometimes throw this balance out of whack, causing the overgrowth of the fungus. In this way, thrush is considered a largely opportunistic condition.
However, it is possible for a person with a large amount of the candida present in the mouth or throat to pass it on to another, so it is better to refrain from kissing until your boyfriend is treated. It may also be passed to the vagina through oral sex.
Please urge him to see a doctor, if he hasn't already done so. The condition is usually cleared up by using an antifungal medication.
Stay well.
